Allow passing a custom TServerEventHandler to ThriftAsyncServerFactory
Allow passing a custom TServerEventHandler to ThriftAsyncServerFactory

Summary:
`ThriftAsyncServerFactory` now accepts `event_handler` which must be a subclass of `thrift.server.TServer`.

If `None` (default), `thrift.server.TServer.TServerEventHandler` is used as before.

This allows to specify a custom handler for custom event handling (e.g. handling new connections to the Thrift server)

Reviewed By: asm89

Differential Revision: D8851304

fbshipit-source-id: 4a418a8dffcc139f092875cf539fd824208fb68a

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