agent: Defer calling assuan_release when it's still in use.
* agent/call-scd.c (struct scd_local_s): Remove LOCK, introduce IN_USE and INVALID flags. (unlock_scd): Call assuan_release when CTX is invalid. (start_scd): Set IN_USE. (agent_scd_check_aliveness): Don't call assuan_release when it's in use.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>