Add naming of databases

Firstly, I'm not a C developer so this might not be the best way to add this feature.  It's not yet ready for merging, I've got a few questions below that I'd like to sort out before it's in a mergeable state.

This adds the ability to name databases:
```
set a 1
dbname site-cache
select 1
set a 1
dbname sessions
info
```
leads to
```
db0:keys=1,expires=0,avg_ttl=0,name=site-cache
db1:keys=1,expires=0,avg_ttl=0,name=sessions
```

The reason for adding this was so I could quickly see at a glance what a database is being used for.

Questions:

1. Would this feature be wanted?
2. How do I go about getting the name to save and persist between restarts?
3. How do I get the name to sync to slaves?