add configuration option to disable database

[msfrank]

certain installations may not write any events locally, instead choosing to forward all events to a remote server. in this case, DatabaseService does not need to be loaded, and libdb (a large package) would not need to be installed. perhaps we should move the terane.db package into terane.outputs.store, so it only is loaded if [plugin:output:store] is defined in the server configuration.

[msfrank]

merged in 814dfae