BUG/MINOR: stats: decrement srv refcount on stats-file release

BUG/MINOR: stats: decrement srv refcount on stats-file release

Servers instance may be removed at runtime. This can occurs during a
stat dump which currently references this server instance. This case is
protected by server refcount to prevent the server immediate release.

CLI output may be interrupted prior to stats dump completion, for
example if client CLI has been disconnected before the full response
transfer. As such, srv_drop() must be called in every stats dump release
callback.

srv_drop() was missing for stats-file dump release callback. This could
cause a race condition which would prevent a server instance to be fully
removed. Fix this by adding srv_drop() invokation into
cli_io_handler_release_dump_stat_file().

This should be backported up to 3.0.