salad: rework bps tree read view API

Currently, there's no notion of a BPS tree read view per se - one can
create an iterator over a regular tree 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 tree as many times as we
want, not just once.

So this patch introduces a concept of bps_tree_view, which contains a
frozen image of a bps_tree and implements a subset of non-modifying
bps_tree methods:

 - bps_tree_view_size
 - bps_tree_view_find
 - bps_tree_view_first
 - bps_tree_view_last
 - bps_tree_view_lower_bound
 - bps_tree_view_lower_bound_elem
 - bps_tree_view_upper_bound
 - bps_tree_view_upper_bound_elem
 - bps_tree_view_iterator_get_elem
 - bps_tree_view_iterator_prev
 - bps_tree_view_iterator_next
 - bps_tree_view_iterator_is_equal

Note, bps_tree and bps_tree_view share bps_tree_iterator, because
iterator methods (get_elem, next, prev, is_equal) take bps_tree or
bps_tree_view. The bps_tree_iterator now contains only block index and
offset.

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:

 - bps_tree_random
 - bps_tree_approximate_count
 - bps_tree_debug_check
 - bps_tree_print

To create a bps_tree_view from a bps_tree, one is supposed to call
bps_tree_view_create. If a bps_tree_view is no longer needed, it should
be destroyed with bps_tree_view_destroy.

Old methods used for creating frozen iterators were dropped:

 - bps_tree_iterator_freeze
 - bps_tree_iterator_destroy

To avoid code duplication, we factored out the common part of bps_tree
and bps_tree_view into a new structure, named bps_tree_common.
Basically, the new structure contains all bps_tree members except
matras, which is stored in bps_tree. The difference between
bps_tree_view and bps_tree 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
bps_tree blocks.

All internal methods now take bps_tree_common instead of bps_tree.
For all public methods that are implemented both for bps_tree and
bps_tree_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 bps_tree_common object
corresponding to a bps_tree_view because of a bug in the bps_tree
implementation, we added !matras_is_read_view_created assertion to
bps_tree_touch_block.

Closes #7191

NO_DOC=refactoring
NO_CHANGELOG=refactoring