ethdb, eth, core: archive old chaindata to s3 compatible bucket

[jmank88]

This PR adds the option to archive old chaindata from leveldb to an s3 compatible bucket. The archive is enabled by setting the following flags:

ARCHIVE OPTIONS:
  --archive value        S3 compatible archive endpoint.
  --archivebucket value  Name of archive bucket. Must already exist.
  --archiveid value      Archive access key ID.
  --archivesecret value  Archive access key secret.
  --archiveage value     Archive age. The number of blocks from head before archiving. (default: 100000)
  --archiveperiod value  How often the archive process runs. (default: 5m0s)

When the archive is enabled, a background process periodically sweeps leveldb for data to archive. Currently this is header, body, and receipts data older than 100000 blocks from the current head.

The archive emits metrics (if enabled) as db/archive/chaindata/*, similar to the existing levedb metrics under db/chaindata/*, for comparison.

#100

[jmank88]

I've been running a local devnet node, and archiving to this bucket: https://cloud.digitalocean.com/spaces/devnet-archive?i=782cce

[jmank88]

Maybe this should be ~120000, for a week of 5 second blocks (#100).

[jmank88]

Removing WIP, since this is stable and behind a feature flag. Needs further testing after merge.