pagetable snapshotting

[spacejam]

on recovery, we should be able to recover a snapshot of the pagetable, then read the log after the pagetable was created for full reconstruction. we only want to read and recover the structure of pages and the offsets of chunks of frags in the log for future retrieval, without pulling in any kv data.

initially this can be done by forking and snapshotting, similar to redis

[spacejam]

ruling out the fork approach. with a large resident set size, forking could take a long time, and then we need to be really careful to avoid allocating memory (assume all mutexes are poisoned after forking). instead, re-use most of the code in PageCache::recover and have a thread write the snapshot without triggering a COW storm.