Refactor sincedb entirely.

[guyboertje]

Now that we have the WatchedFile class to track the state of a file as we know it, I propose to create a WatchedFileStore class in place of the @files hash.
The WatchedFileStore

• should be the responsibility of Watch not Tail.
• should replace the sincedb Hash
• should be loaded from an existing sincedb file with backward compatibility.
• should, when loading, verify that the file can be stated before adding a new watched_file instance.
• should saved in place of the sincedb file
• should be backed by a Hash
• have entries keyed on path and inode but not device
• should find all entries with the same path
• should find all entries with the same inode