vinyl: factor load regulator out of quota

Turned out that throttling isn't going to be as simple as maintaining
the write rate below the estimated dump bandwidth, because we also need
to take into account whether compaction keeps up with dumps. Tracking
compaction progress isn't a trivial task and mixing it in a module
responsible for resource limiting, which vy_quota is, doesn't seem to be
a good idea. Let's factor out the related code into a separate module
and call it vy_regulator. Currently, the new module only keeps track of
the write rate and the dump bandwidth and sets the memory watermark
accordingly, but soon we will extend it to configure throttling as well.

Since write rate and dump bandwidth are now a part of the regulator
subsystem, this patch renames 'quota' entry of box.stat.vinyl() to
'regulator'. It also removes 'quota.usage' and 'quota.limit' altogether,
because memory usage is reported under 'memory.level0' while the limit
can be read from box.cfg.vinyl_memory, and renames 'use_rate' to
'write_rate', because the latter seems to be a more appropriate name.

Needed for #1862