Add configuration to Server to limit number of open databases.

[ecton]

With the initial implementation of the server (#19), a choice was made to delay this work.

The problems this setting is aiming to solve:

• Working within limits of the number of open files
• Sled will be keeping an in-memory cache for each open database. We may want to expose this cache size for configuration, but regardless: each open database will eat memory for cache.

The logic implemented should keep track of the last time a database was accessed, and when a database needs to be unloaded, the oldest database should be evicted first.

One question that needs to be answered is, under high load, if a server is configured to only have, let's say 20 databases open, should we allow temporary bursting if a queue of requests comes in that exceeds that? Or should requests block until the limit is exceeded? Maybe two settings -- a target and a maximum?

[ecton]

This is no longer relevant after #49 is merged. By using one sled instance, the only configuration needed is to expose whatever options we want to allow from sled's configuration.