Under high concurrency / heavy test load (e.g. using run_tests.sh), the
background thread may not get scheduled for a longer period of time. Retry 100
times max before bailing out.
Many profiling related tests make assumptions on the profiling settings,
e.g. opt_prof is off by default, and prof_active is default on when opt_prof is
on. However the default settings can be changed via --with-malloc-conf at build
time. Fixing the tests by adding the assumed settings explicitly.
Adding guarded extents, which are regular extents surrounded by guard pages
(mprotected). To reduce syscalls, small guarded extents are cached as a
separate eset in ecache, and decay through the dirty / muzzy / retained pipeline
as usual.
This change allows every allocator conforming to PAI communicate that it
deferred some work for the future. Without it if a background thread goes into
indefinite sleep, there is no way to notify it about upcoming deferred work.
