user hooks WIP
add Registry.cursor_db class type and corresponding implementation in store
added some TODOs to registry.ml
clean up registry store interface. change Simple_store.delete semantics to no longer throw Not_found.
supply current Interval.t to user functions/hooks
add consistency to user_hook call
nodes shouldn't die when they can't find a user function
fix test_user_function
test user_hook
allow continuing with any client change from user-hook
more powerful user hooks
add fold_(rev_)range helpers for user hooks + feedback from PR
add test for Registry.Cursor_store helper
Merge pull request #379 from Incubaid/user-hooks2

User hooks2