Main changes:
* Change the listing order of files/directories so that the local and remote
snapshots can be compared on-the-fly.
* Introduce a new struct called EntryList that maintains a list of
files/directories, which are kept in memory when the number is lower, and
serialized into a file when there are too many.
* EntryList can also be turned into an on-disk incomplete snapshot quickly,
to support fast-resume on next run.
* ChunkOperator can now download and upload chunks, thus replacing original
ChunkDownloader and ChunkUploader. The new ChunkDownloader is only used
to prefetch chunks during the restore operation.
When copying to an RSA-encrypted storage, we relied on the RSA encryption
version to determine if a chunk is a snapshot chunk or a file chunk. This is
wrong when the source storage is not encrypted or not RSA-encrypted. There
is a more reliable to determine if a chunk is a snapshot chunk or not.
The field encryptionVersion in the Chunk struct is supposed to pass the status
of RSA encrytpion from a source chunk to a destination chunk in a copy command.
This field needs to be a 3-state boolean in order to pass the status correctly.
This is to support public key encryption in the backup operation. You can use
the -key option to supply the public key to the backup command, and then the
same option to supply the private key when restoring a previous revision.
The storage must be encrypted for this to work.
