Save HTTP connection across requests

This influxdb client is not using persistent connections.  This causes a
significant performance loss.  Below is a benchmark script that writes a
single point to influxdb.  Running it takes 44 seconds of time:

    $ ruby -Ilib t.rb
    Writing 1000 points
                       user     system      total        real
    write_points  19.270388   1.394931  20.665319 ( 44.622170)

This works out to a write speed of about 22 points per second, or about
50ms per point.

This is:
* Pretty slow.  If you are generating points quickly maximum resolution
  of 50ms is not great.  A user may lose data because they cannot sample
  quickly enough.
* Pretty inefficient.  For each point written the client library must
  establish a TCP connection, establish a TLS session, finally write
  some data.  Any data written may be restricted by the TCP slow-start
  windowing algorithm.

  Much more ruby code must be run, almost half the time spent is in the
  "user" category, time that could be doing anything else in a ruby
  application, or time that could be used by other processes.

This commit caches HTTP connections across requests.  Running the same
benchmark takes 4.26 seconds:

    $ ruby -Ilib t.rb
    Writing 1000 points
                       user     system      total        real
    write_points   0.551663   0.084603   0.636266 (  4.261201)

This works out to a speed of 234 points per second, or about 5ms per
point.  Writing points now no longer need to recreate a TCP connection,
renegotiate a TLS session, or be held up by TCP window sizing
limitations.

This is much more efficient in terms of CPU time used per point, instead
of ~46% of time taken occurring in the "user" category, now only 13% of
time is in "user".  The balance of the time is now spent waiting for IO
to complete.

Benchmark script:

    require "benchmark"
    require "influxdb"

    n = Integer ENV["N"] rescue 1_000

    influxdb =
      InfluxDB::Client.new url: ENV["INFLUX_URL"],
                           username: ENV["INFLUX_USER"],
                           password: ENV["INFLUX_PASS"],
                           time_precision: "u"

    def write_point counter, influxdb
      points = [
        {
          series: "test",
          values: {
            counter: counter,
          },
        },
      ]

      influxdb.write_points points
    end

    puts "Writing #{n} points"

    Benchmark.bm 12 do |bm|
      bm.report "write_points" do
        n.times do |i|
          write_point i, influxdb
        end
      end
    end

Author: drbrain

lib/influxdb/client/http.rb

@@ -43,13 +43,19 @@ def connect_with_retry
       delay = config.initial_delay
       retry_count = 0
 
-      http = build_http
+      http = get_http
 
       begin
+        http.start unless http.started?
+
         yield http
       rescue *InfluxDB::NON_RECOVERABLE_EXCEPTIONS => e
+        http.finish
+
         raise InfluxDB::ConnectionError, InfluxDB::NON_RECOVERABLE_MESSAGE
       rescue Timeout::Error, *InfluxDB::RECOVERABLE_EXCEPTIONS => e
+        http.finish
+
         retry_count += 1
         unless (config.retry == -1 || retry_count <= config.retry) && !stopped?
           raise InfluxDB::ConnectionError, "Tried #{retry_count - 1} times to reconnect but failed."
@@ -58,9 +64,8 @@ def connect_with_retry
         log(:warn) { "Failed to contact host #{host}: #{e.inspect} - retrying in #{delay}s." }
         sleep delay
         delay = [config.max_delay, delay * 2].min
+
         retry
-      ensure
-        http.finish if http.started?
       end
     end
 
@@ -130,11 +135,23 @@ def generate_cert_store
       store
     end
 
+    def get_http
+      @https ||=
+        begin
+          https = config.hosts.map { |host|
+            build_http host
+          }
+
+          Hash[config.hosts.zip(https)]
+        end
+
+      @https[config.next_host]
+    end
+
     # Builds an http instance, taking into account any configured
     # proxy configuration
-    def build_http
-      host  = config.next_host
-      port  = config.port
+    def build_http(host)
+      port = config.port
 
       http =
         if config.proxy_addr