[Bug fix] mod_tile was not correctly thread-safe in Apache 2.2 and could segfault.

Apache's memory pool operations aren't thread-safe. In get_storage_backend
multiple threads could try to allocate memory on the process pool concurrently
leading to segfaults. To prevent this, wrap that code into a mutex.

Benchmarking seems to indicate, that even though each request has to acquire the lock
this does not appear to have a performance impact.

The issue was reported by Sven Geggus, and diagnosed by Tom Hughes. Thanks.