library: add client mutex lock for multi-thread

library: add client mutex lock for multi-thread

For multi-thread, a mutex lock is added to the client library.
This patch is workaround. Because a function protected by a lock can
be reentered in a callback function. To prevent dead-lock, a mutex
lock should be unlocked when it calls a callback function.

Change-Id: Ic40a6ba0715c5f9bd9f24bc7fd03bc23f1f9519e