Bucket locking

[Foxcapades]

There should be an available feature to lock buckets in a few different ways to help prevent race conditions.

As we (VEuPathDB) intend to follow a "one app, one bucket" rule, this will prevent race conditions for us by default.

For general use this will need to be well documented as for the feature to work, all users of a single bucket will need to follow the same locking rules. Maybe have a default setting using lock files, and allow consumers to change it?

Locks needed:

• write - writes blocked, reads okay
• read-write - reads and writes blocked

Usage of this feature will require the bucket to have holds/locks enabled, and the client will need the s3:PutObjectLegalHold permission