Added couchbase combined storage

Hello!

Inspired by mbtiles storage I've added Couchbase storage to mod_tile (it is actually two memcached storages). One storage is bucket for meta info (path -> md5), second storage is for tiles (md5 -> binary data).

Couchbase is distributed key->value database and it supports memcached protocol.

config string should be as follows:
```
TILEDIR=couchbase:{memcached://localhost:11211,memcached://localhost:11212}
```

Default memcached storage is not suitable for metatiles. Each tile request creates metatile request and 500 rps generate 200 mb/s traffic. So I decided to store each png tile in one key to minimize traffic. Tiles / metainfo ratio is approximately 1/3.

I've also altered meta2tiles utility to convert metatile to couchbase storage.

In addition it is very simple to use nginx-lua for tile serving from couchbase.

One huge disadvantage is no possibility to update old tiles with new ones, as there is no inverted index for md5 -> path value and you can not be sure that updated tile is not used by another path. Actually it is possible to add third bucket for that, but the logic will become more complicated.