filer: session lease + reaping for POSIX locks (#9666)

* filer: session lease + reaping for POSIX locks

A mount renews its session lease by keepalive (new KEEP_ALIVE op); the
owner filer records last-seen per session and a background sweeper reaps
the locks of leased sessions that stop renewing — a dead or partitioned
mount. Only sessions that have renewed are leased, so this is inert until
mounts run with -posixLock.

* mount: route POSIX advisory locks to the owner filer (-posixLock) (#9665)

mount: route POSIX advisory locks to the owner filer under -dlm

With -dlm, GetLk/SetLk/SetLkw and the flush/release cleanup paths go to
the inode's owner filer via the PosixLock RPC instead of the local table,
so flock/fcntl are honored across mounts. Advisory locking rides the same
switch as whole-file write coordination — and is therefore off under
writeback cache, which implies single-writer. The mount calls its filer
and relies on filer-side forwarding to reach the owner. Keys are the inode
identity (HardLinkId else path); SetLkw is client-side polling with the
FUSE cancel channel (no server wait queue); a per-mount session id
namespaces owners; a local hint avoids a release RPC on every close.

* mount,filer: bound posix-lock release RPCs and stop the reaper on shutdown

The unlock/release RPCs run off the syscall path (close/flush) and used
context.Background() with no deadline, so a slow or unreachable filer could
hang close() indefinitely; bound them to 5s (they still aren't cancelled by
an interrupt). The lease-reaping sweeper now selects on a stop channel that
FilerServer.Shutdown closes, instead of looping for the process lifetime.

other/java/client/src/main/proto/filer.proto

@@ -396,6 +396,7 @@ enum PosixLockOp {
     GET_LK = 2;             // report a conflicting lock, if any
     RELEASE_POSIX_OWNER = 3; // drop the owner's fcntl locks (flush-time)
     RELEASE_FLOCK_OWNER = 4; // drop the owner's flock locks (release-time)
+    KEEP_ALIVE = 5;         // renew the session's lease on this owner (lock.sid)
 }
 
 message PosixLockResponse {