XDCR - XDCR/Mobile Coexistence Optimization: Avoid body decompression multiple times over the lifetime of a mutation in XDCR

Right now, we decompress the body in the following places:

1. Router: If filter is specified which concerns the document body.
2. Xmem: For upadating the xattrs before sending to target.

A potential performance optimization is to avoid decompressing multiple times and cache the decompressed value, the first time decompression is done.