Correctly shutdown wrapped Executor in PausableChannelEventExecutor.shutdown()

Motivation:

PausableChannelEventExecutor().shutdown() used to call unwrap().terminationFuture() instead of unwrap.shutdown().
This error was reported by @xfrag in a comment to a commit message [1]. Tyvm.

Modifications:

PausableChannelEventExecutor.shutdown() now correctly invokes unwrap().shutdown() instead of unwrap().terminationFuture().

Result:

Correct code.

[1] https://github.com/netty/netty/commit/220660e351b2a22112b19c4af45e403eab1f73ab#commitcomment-8489643
Rewrite HttpObjectDecoder to make use of proper state machine

Motivation:

HttpObjectDecoder extended ReplayDecoder which is slightly slower then ByteToMessageDecoder.

Modifications:

- Changed super class of HttpObjectDecoder from ReplayDecoder to ByteToMessageDecoder.
- Rewrote decode() method of HttpObjectDecoder to use proper state machine.
- Changed private methods HeaderParser.parse(ByteBuf), readHeaders(ByteBuf) and readTrailingHeaders(ByteBuf), skipControlCharacters(ByteBuf) to consider available bytes.
- Set HeaderParser and LineParser as static inner classes.
- Replaced not safe actualReadableBytes() with buffer.readableBytes().

Result:

Improved performance of HttpObjectDecoder by approximately 177%.
Benchmark for HttpRequestDecoder