Fix reference count leak when mmapping pages

When opening readonly, mmap readonly

Don't destroy mmap_allocator on failure to open file

Add pb_mmap_open_action_append to mmap buffer

Don't forget pagebuf_protected.h during install

Add protected functions to pb_trivial_buffer operations

Update fullclean script

Remove public access to increment_data_revision function

Small tidy up in pagebuf_mmap headers

Clone page before overwriting if not exclusively owned

Re-structure allocators:

Update TODO

Move definitions of pb_page and pb_buffer_insert to protected level

Re-order some of the buffer::iterator types

TODO update