Add flushing support to fasthttpadaptor (#2054)

* Add flushing support to fasthttpadaptor

* refactor(fasthttphandler): Fix comment typos

* refactor(fasthttphandler): Fix early closing of net/http handler

* refactor(fasthttphandler): Apply requested changes

* refactor(fasthttphandler): Reduce memory allocations by using sync.Pool

* refactor(fasthttphandler): Fix improper releaseNetHTTPResponseWriter

* refactor(fasthttphandler): Add buffer sync.Pool with panic assert

* refactor(fasthttphandler): Fix hijacked-related response writer race condition

* refactor(fasthttphandler): Rename bufW to bufRW

* refactor(fasthttphandler): Ensure proper responseMutex use

* refactor(fasthttphandler): Specify minBufferSize to ensure reading 32 KiB chunks in streaming mode

* refactor(fasthttphandler): Fix release logic

* refactor(fasthttphandler): Fix handlerConn race condition

* refactor(fasthttphandler): Explicitly ignore handlerConn close error

* refactor(fasthttphandler): Use sync.Once, sync.Cond, and a single channel for mode management

* refactor(fasthttphandler): Remove commented code

* refactor(fasthttphandler): Add period to respect linter

* refactor(fasthttphandler): Remove return else clauses to respect lint

Author: grivera64