Things not done because probably disruptive:
* kmod filesystem string
* sysctl/debugfs/trace
* metrics names
* xmon instance names
Some of these might be renamed too, but starting with a relatively
safe set.
We do not initialise the failure domain field ans as a result memcp
evaluates to false triggering false positives in blockservice changes.
The effect is more than 10x slowdown of file reads.
Also added tracepoints which were useful in troubleshooting this.
This saves quite a bit of space in the span structure. I'm doing this
in preparation for stripe-level span cache, since I plan to just tie
the span metadata lifetime to the inode lifetime, so space savings
in `struct eggsfs_span` are a good thing.
I think it also makes good sense, block service updates are propagated
instantly.
