overhaul checker invocation

overhaul checker invocation

Removed the Checker service, removed checker results storage (both in-memory
and the tiny stub of sqlite-based storage). Added ICheckable, all
check/verify is now done by calling the check() method on filenodes and
dirnodes (immutable files, literal files, mutable files, and directory
instances).

Checker results are returned in a Results instance, with an html() method for
display. Checker results have been temporarily removed from the wui directory
listing until we make some other fixes.

Also fixed client.create_node_from_uri() to create LiteralFileNodes properly,
since they have different checking behavior. Previously we were creating full
FileNodes with LIT uris inside, which were downloadable but not checkable.