consortium-v2/snapshot: prune old, unused snapshots

[Francesco4203]

Only some most recent consortium snapshots are used, so this PR is to prune some old, unused snapshots. The pruning can be done in two ways:

• Periodically: every day, automatically prune all the snapshots from 6 to 7 days ago
• Manually: command to prune all the snapshots, keeping only some most current snapshots

Example command to prune all snapshots, keeping 1000 latest ones:

./build/bin/ronin db prune-consortium-snapshot --snapshot.keep-after-pruning=1000 --datadir="path-to-datadir"

[huyngopt1994]

@Francesco4203 Could u give some description related to its usage?

[Francesco4203]

@huyngopt1994 I have updated the description