[mod_tile] use a separate storage backend per thread in apache 2.4

Storage backends might not be fully thread safe when reusing the same context, like for example the memcached storage backend,

therefore initialise a separate storage backend / context per thread rather than per process.

Unfortunately there is no way to do this in Apache 2.2. So there you will need to use mpm prefork for non thread safe backends



git-svn-id: http://svn.openstreetmap.org/applications/utils/mod_tile@29420 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4
Fix stability issues with rados backend



git-svn-id: http://svn.openstreetmap.org/applications/utils/mod_tile@29421 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4