Now we read the revision count on-demand, trading off some extra reads
for a smaller lfsr_rbyd_t struct.
I believe this is worth it because:
1. We're created a lot of lfsr_rbyd_t structs as a part of the relatively
complicated mdir/btree commit logic in order to safely fallback on errors.
2. We don't really need the revision count for our Cow btrees, so we
only need to read the revision count on mdir fetch (which we were
already reading too many times), on mdir compact, and on rbyd fetch
as a part of checksum calculation.
This really only adds a O(1) cost when we are compacting, which is rather
small.
Current measurements:
code: 8980 -> 9036 (+0.6%)
stack: 1024 -> 1000 (-2.3%)
Though note this is currently without any mdir/btree commit code being
dragged in.
Christopher Haster
0690a86f1d