Currently, region_group forms a hierarchy. Originally it was a tree,
but previous work whittled it down to a parent-child relationship
(with a single, possible optional parent, and a single child).
The actual behavior of the parent and child are very different, so
it makes sense to split them. The main difference is that the parent
does not contain any regions (memtables), but the child does.
This patch mechanically splits the class. The parent is named
memory_hard_limit (reflecting its role to prevent lsa allocation
above the memtable configured hard limit). The child is still named
region_group.
Details of the transformation:
- each function or data member in region_group is either moved to
memory_hard_limit, duplicated in memory_hard_limit, or left in
region_group.
- the _regions and _blocked_requests members, which were always
empty in the parent, were not duplicated. Any member that only accessed
them was similarly left alone.
- the "no_reclaimer" static member which was only used in the parent
was moved there. Similarly the constructor which accepted it
was moved.
- _child was moved to the parent, and _parent was kept in the child
(more or less the defining change of the split) Similarly
add(region_group*) and del(region_group*) (which manage _child) were moved.
- do_for_each_parent(), which iterated to the top of the tree, was removed
and its callers manually unroll the loop. For the parent, this is just
a single iteration (since we're iterating towards the root), for the child,
this can be two iterations, but the second one is usually simpler since
the parent has many members removed.
- do_update(), introduced in the previous patch, was made a template that
can act on either the parent or the child. It will be further simplified
later.
- some tests that check now-impossible topologies were removed.
- the parent's shutdown() is trivial since it has no _blocked_requests,
but it was kept to reduce churn in the callers.
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