mempool: fix page_range_allocator::_bitmap resize

When resizing a dynamic_bitset some memory may be freed. In such case it is
not safe to attempt to merge unallocated block since the bitmap is in
undefined state. Instead, the memory free is deferred to a moment when the
resize is done and bitmap is reinitialized with up to date values.

page_range_allocator::_bitmap uses now custom allocator to get rid of
numerous checks agains _nobitmap in allocation and deallocation code.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Revert "Revert "mempool: use new page range allocator""

This reverts commit 0ff46442658062554221fc22f7bf2225b2f9b4b0.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>