Fix compilation warning in storage manager to get rid of 'initialzer lists' warning

Fix compilation warning in storage manager to get rid of 'initialzer lists' warning

Description:
when using:
mutex = PTHREAD_MUTEX_INITIALIZER;
instead of:
pthread_mutex_init(&mutex, NULL);
There could be warning on some environments:
store_mngr.c:31:10: warning: extended initializer lists only available
with -std=c++11 or -std=gnu++11 [enabled by default]
    mutex = PTHREAD_MUTEX_INITIALIZER;

To get rid of it, simply use the init/destroy mutex helpers.