lib: add a wrapper for pthread_mutex_trylock()

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: use mutex instead of busy waiting for object pushing control

This patch removes a confusing variable "in_push" of object cache. The
most important change is removing a usleep() wait based on the
variable.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>