v0.6.0

One new argument, at chunkmirage's request. No placement change and no unit table change;
nothing changes for a caller that does not pass it.

• wa.shared_context(cache_bytes, threads=): None (the default) limits the decoding and
  file I/O pools to thread_budget() as in 0.5, a positive number limits them to that many,
  and 0 writes no limit, so tensorstore sizes them itself (every CPU on the node). Each
  (cache_bytes, threads) is one watched context for the process, so a server can share
  one cache pool across every store it opens at the thread count it chose, without building
  and watching its own context. A pickled handle on such a context comes back on the same
  one; threads=None still lets the other process size by its own job.
• Documented what a pool of one thread costs (spec Behaviour 7, capped_context_spec):
  nothing on one-chunk blocks; on 8-chunk blocks, half the speed of 4 in a two-slot job
  but only 7 to 19% slower in a one-slot one (c-elegans on /nrs), so a worker per slot
  loses little at the default.