Optimized multipart download performance

[snej]

(Please see #1149. I'm just filing this issue as a pointer to it, for the benefit of our release-notes generator script, which only looks at issues not PRs.)

From @robertjpayne :

The CBLMultipartReader is somewhat slow parsing on older devices. This is almost entirely caused by the [CBLMultipartReader deleteUpThrough:] API, mostly because this particular NSMutableData API is very slow though even a realloc here wouldn't be that great.

This slowness is easily observable on an older device ( iPhone 4S in my case ) with instruments time profiler.