Added couchbase combined storage

Hello!

Inspired by mbtiles storage I've added Couchbase storage to mod_tile (it is actually two memcached storages, I'll add libcouchbase in future). One storage is bucket for metahash info (path -> stat+md5*64), 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.

I've also altered meta2tiles utility to convert metatile to couchbase metahash storage.

In addition it is very simple to use nginx-lua for tile serving from couchbase.