doc: Change listed RBD cache defaults to actual defaults

Signed-off-by: Adam Crume <adamcrume@gmail.com>
osdc: Fix ObjectCounter::_readx perf counters

Signed-off-by: Adam Crume <adamcrume@gmail.com>
librbd: Add read-ahead

Signed-off-by: Adam Crume <adamcrume@gmail.com>
client: Replace client readahead logic with Readahead

Signed-off-by: Adam Crume <adamcrume@gmail.com>
Merge remote-tracking branch 'origin/wip-rbd-readahead'

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

Conflicts:
	src/client/Fh.h