Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

Prepare to make LightSong non-assignable.

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

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

Simplify the definition of many build options.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

The new method IsPlugin() replaces the "is_simple" flag.

Move from db/DatabaseGlue.cxx, eliminating global variable.