Mark and Sweep GC for Storage

[mikera]

It should be possible to do a concurrent mark+sweep on the storage file to discard old data

Rough Procedure:

• Set a "marking" flag (which will cause new additions to be automatically marked)
• Mark all values to retain (at Peer's discretion)
• Copy marked values to new storage file
• Atomically switch Peer to using the new storage file, with cleared marking flag
• Delete / archive old storage file as desired