Cache check mount calls

Cache check mount calls to avoid lots of stat system calls at high
request loads. The amount of time to cache the mount checks is
configurable via the /etc/swift.conf parameter (section [swift-disk]):

    check_mount_cache_time = 10

Note that this parameter affects *all* mount check behaviors across
the account, container and object servers, so the parameter lives in
swift.conf as a result.

Change-Id: I7d0fa05544b85641b36ddfeeadfd3b85a87e20d9
Signed-off-by: Peter Portante <peter.portante@redhat.com>