Don't use apr_global_mutex to guard storage backend initialisation

[apmon]

Commit 2c538916be4f8dff28181ea5a47d65d84b4b2a03 introduced locking around memory allocation for an array containing pointers to the storage backends, as apache's memory pool functions aren't thread safe. It used apr_global_mutex, but I think apr_global_mutex not only synchronizes between threads, but also between process which isn't necessary for the storage backend memory allocation. Instead presumably apr_thread_mutex is what we want.

I need to verify this before committing the change, hence this reminder ticket.