ntdb: enhancement to allow direct access to the ntdb map during expansion.

ntdb: enhancement to allow direct access to the ntdb map during expansion.

This means keeping the old mmap around when we expand the database.
We could revert to read/write, except for platforms with incoherent
mmap (ie. OpenBSD), where we need to use mmap for all accesses.

Thus we keep a linked list of old maps, and unmap them when the last access
finally goes away.

This is required if we want ntdb_parse_record() callbacks to be able
to expand the database.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>