Add support for pluggable storage backends to mod_tile / renderd

So far, (meta)tiles could only be stored and retrieved from a locally mounted posix filesystem.  Although, the posix
filesystem API is it self a plugable storage layer into which one can mount many different filsystems, from temporary
filesystems to network filesystems, as mod_tile installations scale up to multi-server environments this might not be sufficient and one
may want to use other storage layers.

This patch therefore abstracts all storage calls out into a separate API that can be implemented by various storage backends to fit the needs of different
installations. 

Three storage backends are included in this commit:

- file backend: This is the equivalent of what existed before in mod_tile / renderd. This uses a posix filesystem to store (meta)tiles
- memcached: This stores tiles in a memcached store
- rados: This stores tiles in a rados / ceph cluster

The memcached and rados backends should currently still be considered as experimental

There are also other refactoring and cleanups in this commit



git-svn-id: http://svn.openstreetmap.org/applications/utils/mod_tile@29389 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4