A bunch of changes, most in response to Peter's review.

A bunch of changes, most in response to Peter's review.

Logger:
Edit() and Record() have been replaced with the single Update() method which takes a function as input (suggested by Tom).
lastWrite replaced by nextWriteAllowed, for cleaner logic
Added writeScheduled to reduce the number of writes scheduled and attempted, thereby reducing lock contention
Added sanity-checking of params
A bunch of overdue cleanup
Update documentation to reflect the above changes

Manifest:
Renamed ManifestLine to ManifestStream

Util:
Deleted a lot of crap that proved less useful than I thought.
Moved collection.NumberCollectionsAvailable() to util.NumberItemsAvailable() and made it more generic.

collection:
Just cleanup in response to changes in above packages.

keep:
Switched Mtime from int to int64 to avoid y2038 problems.
Switched approach for avoiding keep proxy from using "accessible" to filtering on service_type = disk.
Cleanup in response to changes in above packages.

loggerutil:
Cleanup in response to changes in logger.