common/shared_cache.hpp: avoid list::size()

Signed-off-by: Samuel Just <sam.just@inktank.com>
common/shared_cache.hpp: compact to a single lookup where possible

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore: perform LFNIndex lookup without holding fdcache lock

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