Client creation is not threadsafe

[robbavey]

The construction of the Manticore Http Client is not threadsafe - the client is not constructed until first used in the multi_receive method, where it makes a call to the client method in the http client mixin.

With some debug logging adding, we can see:

[2021-04-30T17:20:38,734][[main]>worker0][INFO ][logstash.outputs.http    ][main][http_out] created #<Manticore::Client:0x4eb5309d>
[2021-04-30T17:20:38,734][[main]>worker3][INFO ][logstash.outputs.http    ][main][http_out] created #<Manticore::Client:0x165a8c91>
[2021-04-30T17:20:38,735][[main]>worker1][INFO ][logstash.outputs.http    ][main][http_out] created #<Manticore::Client:0x2d81234c>
[2021-04-30T17:20:38,735][[main]>worker2][INFO ][logstash.outputs.http    ][main][http_out] created #<Manticore::Client:0x3258927d>
[2021-04-30T17:20:40,853][[main]>worker1][INFO ][logstash.outputs.http    ][main][http_out] Used #<Manticore::Client:0x3258927d> to send request
[2021-04-30T17:20:40,853][[main]>worker3][INFO ][logstash.outputs.http    ][main][http_out] Used #<Manticore::Client:0x3258927d> to send request
[2021-04-30T17:20:40,854][[main]>worker2][INFO ][logstash.outputs.http    ][main][http_out] Used #<Manticore::Client:0x3258927d> to send request
[2021-04-30T17:20:40,854][[main]>worker0][INFO ][logstash.outputs.http    ][main][http_out] Used #<Manticore::Client:0x3258927d> to send request