Fix a bunch of memory problems in beagle:

Fix a bunch of memory problems in beagle:

* Fix the usage of assembly.GetTypes() - Mono never GCs Type objects, and
calling GetTypes() instantiates evrey one of them contained in an
assembly.  Paired with some code that walked all the loaded assemblies,
we were instantiating thousands of types unnecessarily that stayed around
forever.

* Remove ReflectionFu methods that get all types that subclass from a type
or implement an interface; methods that do this should find a different
way.

* Implement assembly-scoped attributes for all of our dynamically loaded
types, including Queryables, Filters, Tiles, RequestMessages,
ResponseMessages, RequestMessageExecutors, and components in the
Thunderbird backend.  This means that these types now have to register
themselves to be loaded, but they can still be dynamically loaded from
outside assemblies.

* No longer run mono with --debug unless the --mono-debug variable is
passed to beagled.  This saves us some memory off the top, but there is
also some evidence that Mono leaks memory when this is turned on.

For me, these changes result in about a 40% reduction in startup memory.