Separate boss and worker group in `ServerBootstrap`.

[mashhurs]

Description

Recently, we observed that using same event loop executor group for both boss and worker seems brings problems for heavily used systems. When terminating the plugin, event loop group still may receive incoming connection.

Acceptance expectation

Validate the behavior, separate the groups and terminate boss group first.

Related:

• In a reality event loop wasn't terminated, might be event handler thread loop was full When using Logstash http input plugin the connection not closed so that the fd process increased  #175
• When executor gets terminated but still tasks in netty queue and chanell is handling in channelRead0, this exception raises RejectedExecutionException NettyHttpServer.run #131