Metamorph: be less aggressive about closing readers right away

If the last plane has been read, or more than 128 readers are open, then
we can close the reader; otherwise, keep it open to prevent
reinitializing a reader on every openBytes call.

Fixes #11968.
Merge pull request #1102 from melissalinkert/rebased/develop/metamorph-performance

Metamorph .nd performance improvements (rebased onto develop)