Improve mbtiles diff validation, add new metadata field, usability

[nyurik]

Current state

• Files generated by martin's mbtiles tool contain agg_tiles_hash metadata field. The field represent an aggregate hash of all tiles table (z/x/y/data). See docs.
• When diffing two mbtiles files - file A and file B, the generated delta file will include agg_tiles_hash_after_apply value - the hash of the file B.

Proposal

Current design has a few shortcomings that need to be addressed. The biggest issues is that any mbtiles can be applied to any other mbtiles - which is different than "merge-copying" (simply adding content to the same file from multiple other files, where tiles with the same x/y/z override existing ones.

• Add a new metadata field agg_tiles_hash_before_apply (name?) to the delta files -- this corresponds to the agg_tiles_hash of thefile A.
• Improve logic and default behavior

Creating a diff file between A and B

• Refuse to work if either A or B files contain agg_tiles_hash_before_apply or agg_tiles_hash_after_apply (unless there is a --force flag)
• If either files A or B do not contain agg_tiles_hash, compute it (but don't save it?)
• Store agg_tiles_hash as agg_tiles_hash_before_apply and agg_tiles_hash_after_apply values in the delta file

Patch file A with a diff, or apply on copy

• Refuse to work if file A contain agg_tiles_hash_before_apply or agg_tiles_hash_after_apply
• Refuse to work if the diff file does NOT contain agg_tiles_hash, agg_tiles_hash_before_apply or agg_tiles_hash_after_apply
• Unless some --validate flag is set, accept agg_tiles_hash stored in file A as valid. If it does not exist, compute it.
• Refuse to work if the stored/computed agg_tiles_hash does not match agg_tiles_hash_before_apply
• After patching file A, validate the file against the agg_tiles_hash_after_apply value