Use walk!=nullptr instead.

Necessary for "final" on gcc 4.6.

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.

Prepare for the plugin interface.

Keep only a reference.

Don't use the global variable "instance".

Don't use the global variables from the DatabaseSimple library.

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