BUG/MINOR: pools: don't mark the thread harmless if already isolated

BUG/MINOR: pools: don't mark the thread harmless if already isolated

Commit 85b2cae63 ("MINOR: pools: make the thread harmless during the
mmap/munmap syscalls") was used to relax the pressure experienced by
other threads when running in debug mode with UAF enabled. It places
a pair of thread_harmless_now()/thread_harmless_end() around the call
to mmap(), assuming callers are not sensitive to parallel activity.
But there are a few cases like "show sess all" where this happens in
isolated threads, and marking the thread as harmless there is a very
bad idea, even worse when arriving to thread_harmless_end() which loops
forever.

Let's only do that when the thread is not isolated. No backport is
needed as the patch above was only in 2.1-dev.