If the operating system will allocate private pages on demand for the
pagemap then use the FlatPageMap by default as it generates better code
for deallocation.
The PAL can now advertise that it supports aligned allocation. If it
does not, then the memory provider will do the alignment for it.
This change still leaves the PAL responsible for systematic testing, but
it should now be much easier to lift that out.
This does not deallocate memory until the OS tells us that we are short
on memory, then tries to decommit all of the cached chunks (except for
the first page, used for the linked lists).
Nowhere near enough testing to commit to master yet!
