salad: rework light hash read view API

Currently, there's no notion of a LIGHT hash table read view per se -
one can create an iterator over a regular hash table and then "freeze"
it. This works just fine for snapshotting and joining replicas, but this
spartan API doesn't let us implement user read views, because to do that
we need to do lookups and create iterators over a frozen hash table as
many times as we want, not just once.

So this patch introduces a concept of LIGHT(view), which contains a
frozen image of a LIGHT(core) and implements a subset of non-modifying
LIGHT(core) methods:

 - LIGHT(view_count)
 - LIGHT(view_find)
 - LIGHT(view_find_key)
 - LIGHT(view_get)
 - LIGHT(view_iterator_begin)
 - LIGHT(view_iterator_key)
 - LIGHT(view_iterator_get_and_next)

Note, LIGHT(core) and LIGHT(view) share LIGHT(iterator), because
iterator methods (begin, key, get_and_next) take LIGHT(core) or
LIGHT(view). The LIGHT(iterator) now contains only a hash table slot.

We could also implement the rest of non-modifying methods, but didn't do
that, because they are not needed to implement user read views:

 - LIGHT(random)
 - LIGHT(selfcheck)

To create a LIGHT(view) from a LIGHT(core), one is supposed to call
LIGHT(view_create). If a LIGHT(view) is no longer needed, it should be
destroyed with LIGHT(view_destroy).

Old methods used for creating frozen iterators were dropped:

 - LIGHT(iterator_freeze)
 - LIGHT(iterator_destroy)

To avoid code duplication, we factored out the common part of
LIGHT(core) and LIGHT(view) into a new structure, named LIGHT(common).
Basically, the new structure contains all LIGHT(core) members except
matras, which is stored in LIGHT(core). The difference between
LIGHT(view) and LIGHT(core) is that the latter stores matras_view
instead of matras. The common part contains pointers to matras and
matras_view, which are used by internal implementation to look up
LIGHT(record).

All internal methods now take LIGHT(common) instead of LIGHT(core).
For all public methods that are implemented both for LIGHT(core) and
LIGHT(view), we have the common implementation defined in _impl suffixed
private function, which is called by the corresponding public functions.

To ensure that a modifying method isn't called on LIGHT(common) object
corresponding to a LIGHT(view) because of a bug in the LIGHT code, we
added !matras_is_read_view_created assertion to LIGHT(touch_record),
LIGHT(prepare_first_insert), and LIGHT(grow).

Closes #7192

NO_DOC=refactoring
NO_CHANGELOG=refactoring