In plugin support `plugin/container/handler.go` code responsible for reading plugin protocol requests from the socket handles each request in sequence, creating a hard limit to the amount of item key requests the plugin is able to handle. 

This means that long-running item requests and failing requests, blocks the execution of others.

Investigate and propose a plan to support concurrent item request execution in external plugins.

Things we already know: some of the plugin protocol requests must remain sequential and even block others until they have finished. Requests like Start, Configure (posibely more) need to be executed in sequence and execute requests can't run before these have finished, as plugin can' execute item key requests until it has config, with sessions (and other stuff).

Additionaly check all SQL external plugin connection cache mutex handling and improve its concurrency performance, like it is done in https://git.zabbix.com/projects/ZBX/repos/zabbix/pull-requests/7249/overview