it: added subrotines for iterator state mgmt
it: added stubs for iterator actions
it: simplified list mgmt routines

Also added dnet_iterator_list_next_id_nolock() for finding next
available id for iterator.
it: filled dnet_iterator_set_state stub

Also simplified code an removed unused variables.
it: moved dnet_iterator_list_next_id_nolock

Also added failsafe assert.
it: init iterator_list at start
it: manage iterator state in elliptics
it: python: added symbolic constants
it: python: exported id and action
it: python: formatted IteratorRequest
test: exit with 1 on fail
it: use uint32_t instead of enum (for now)
test: set data flag on iteration
it: improved logging
test: rework iterator test, added flow control
it: allow flow control of iterators
it: improve logging
it: move flow control to separate subroutine
it: added sanity and comments
it: prettify comments