Fix memory leak in BA mount service

Fix memory leak in BA mount service

The cache used in BA mount service uses weakKeys() so the keys (DOM mountpoint)
could be GCed when no ono, except cache, referenced them).
However the values in the cache (BA mountpoint) also kept a reference to
the key, so the key was never weakly reachable and thus never GCed
resulting in a memory leak.

Which is especially visible in case of frequent reconnects of netconf
mountpoints where after every reconnect a new DOM mountpoint is created,
requiring a new BA mountpoint creating new entry in the cache of BA mount service...

Added also a simple test verifying proper cache cleanup

Change-Id: I9c109dd6d499d6185842d6a1ad5a59d75565af5c
Signed-off-by: Maros Marsalek <mmarsalek@frinx.io>