Implement multiple database instances support

[danielealbano]

Support for multiple databases should be implemented as a parallel layer checked together after a key is checked out.

There are several options that needs to be explored:

• a 1 byte can be appended or prepended to the key to indicate the database number, in this way the hashtable internally would always compare the key including the database number
• the database number should also be stored in the storage db entry index structure
• every storage_dbop* should request the database number
• the database number should be stored in the connection context
• the max number of database should be configurable
• the initial database number should be configurable