doc/dev/osd_internals: add some info about throttles

Signed-off-by: Samuel Just <sam.just@inktank.com>
common/shared_cache.hpp: fix set_size()

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore: add fd cache

Signed-off-by: Samuel Just <sam.just@inktank.com>
os/: Add WBThrottle

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore: integrate WBThrottle

Signed-off-by: Samuel Just <sam.just@inktank.com>