Unify DynamicTables in HPACK and QPACK

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There is a lot of similarity in the both Dynamic Tables, and using MIMEHdr doesn't seem like a great idea (lots of overhead). We should implement a DynamicTable that can be used for the both HPACK and QPACK without using MIMEHdr.

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I have a new implementation that works for HPACK. It was originally wrote for QPACK and I made a few modification for HPACK (and fixed some bugs). Benchmark result on my laptop looks good (a bit faster than current HPACK). I haven't tested it with QPACK, but QPACK implementation itself currently does not work. We should probably do QPACK integration on a separate issue/PR.