Add IOBuf::cloneCoalesced()

Add IOBuf::cloneCoalesced()

Summary:
`LZ4Codec::doCompress()` and `doUncompress()` need to coalesce a `const IOBuf*`, so they `clone()` and then `coalesce()`.
It is more efficient to do this in one step, as shown by the benchmarks.

Reviewed By: yfeldblum, simpkins

Differential Revision: D4727802

fbshipit-source-id: f8d5f6187f2ee804301232b3c75b848330169fc5