Closes #258

Fixes "search already in progress" errors.

Allows database plugins to use the EventThread, e.g. for CURL
integration.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

See https://picard.musicbrainz.org/docs/mappings/

This tag is useful when the user would like all releases of the same
album to be sorted next to each other.

If the base class is not accessible, the "catching" the base class
won't work.  This caused the fatal error:

 terminate called after throwing an instance of 'LibmpdclientError'

Implementations shall use exceptions instead.

Use this function instead of CheckError() when we already know an
error has occurred.

The old struct config_param remains only for top-level string options.