Improve mount performance

[hayley-leblanc]

SquirrelFS currently takes longer to mount than other PM file systems; we should add optimizations to improve this.

Possible optimizations:

• Perform system scan/index rebuild in parallel.
• Serialize indexes/allocators and store them durably during clean unmount so they do not have to be fully reconstructed. We probably want to use a checksum/crash bit to ensure that the serialized structures are consistent and correct before using them.
  • This won't work in the event of a crash; we'll still need to do the full disk scan then.
• Reduce the size/complexity of in-memory data structures to reduce the amount of time it takes to set them up at mount.