Proposal for a snapshot scheduling API

[vladtr]

This PR contains proposal for a 3 new additions to a snapshot API that would allow:

• schedule a new recurring snapshot task
• retrieve all schedculed snapshot requests with associated pending snapshots
• un-schedule snapshot task

Please refer to the following issue for additional details:
AntelopeIO/leap#396

[linh2931]

Can status show which ones are in progress, which ones are pending? For in progress, can the number of blocks being done shown?

[arhag]

A pending snapshot is one in progress. Do you see a distinction between the two?

[linh2931]

OK, I see.

[arhag]

Changes to still make:

• Remove pagination.
• Remove snapshot_request_time.
• Remove pending snapshots from JSON data model and instead figure out mapping on startup by enumerating pending snapshots in snapshot directory (following filename schema). Read snapshot to get block number and block ID (and verify with filename values) as well as head_block_time and snapshot version number.