KNF fixes and comment typo.
Name async_new_op async_flush using underscore (not camelcase) convention.
Fixes for Java async stability and performance.

Cache field ids and method ids for all async operations.  Invalidate
unpackers by setting fields, rather than calling a method to do it.

Use javaClose to invalidate the swigCptr for a AsyncOp at the end of
the callback.  Any use of an AsyncOp after its lifetime has ended will
give a NullPointerException.

Declare that new_async_op throws WiredTigerException, which happens
when there are no more slots available.

Added a test with a million items that typically does no pauses when
creating ops, but catches WiredTigerException and retries with a short
sleep.