Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

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.

Keep only a reference.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

The Song class is only used for database songs now.  A Song without a
Directory is not possible anymore.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

Instead of using the first DecoderPlugin that supports the suffix, use
the first one that actually implements the "container_scan" method.

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".