When building with ACL2_MT_MEMO=1 and ACL2_PAR=p, turn multithreading
memoization on by default, since that's safer.

Insert limit on the number of "ignoring time increment" messages that
will be printed, in case of horribly incorrect timings.

Expand memoize tests with some multithreaded tests.  Kind of a mess
because I was debugging issue 264.