BUG/MINOR: stats: drop srv refcount on early release

BUG/MINOR: stats: drop srv refcount on early release

Server refcount is used to protect from server deletion while dumping a
server instance, for stats dump on both CLI and HTTP applet. However,
dump can be aborted prematurely before reaching the end. In this case,
server refcount is never decremented.

This bug can cause an inconsistency on servers refcount, preventing them
to be deleted even after "del server" success.

To fix this, implement release handler for both stats CLI and HTTP
applet. Drop server reference if dump was interrupted during servers
loop.

This should be backported up to 2.6.