tracker-extract: Cache TrackerConfig::max-bytes

This is the only setting read from extractor modules. It turns out
that under the right circumstances a g_settings_get_value() call
can trigger open() calls with readwrite permissions from the caller
thread. This is forbidden in extractor threads and triggers warnings
like:

dconf-CRITICAL **: unable to create file '/run/user/1000/dconf/user':
Permission denied.  dconf will not work properly.

Also, since this setting is meaningless to update during tracker-extract
lifetime, just drop the g_settings_bind() call, this means the setting
is only read once on startup.

https://bugzilla.gnome.org/show_bug.cgi?id=779342