regionlists are updated during DLE submit under the logmtx. Thus, we
should avoid synchronous memory allocations.

We change the strategy as follows:

* Memory for the top regionlist (which has one regl embedded) _is_
  allocated synchronously, but with maximum cram to reduce latencies
  at the expense of memory efficiency.

  The case where the allocation does block will not hit us for the
  most critical path in fellow_log_dle_submit(), because we
  pre-allocate there outside the logmtx.

* When we create the top regionlist, we make two asynchronous memory
  allocation requests for our hard-coded size (16KB for prod), one
  crammed and one not. The crammed request is made such that we get
  _any_ memory rather than waiting.

* When we need to extend the regionlist, we should already have an
  allocation available (if not, we need to wait, bad luck). The next
  allocation available is either [1] (uncrammed) left over after the
  previous extension, or [0], which is potentially crammed. If it is
  and we have an uncrammed [1], then we use that and return the
  crammed allocation. If there are no allocations left, we issue the
  next asynchronous request.