Feature: allow merger to read from multiple one-block locations

[matthewdarwin]

It appears (but is not proven), than in our environment when the s3 storage server is under extreme load, that one-block files go missing. The working theory is that having multiple deep-mind processes writing to the same file to the same location causes some race condition and the file just disappears.

Possible workaround: have each of the mindreaders write to its own location and then have the merger read from all those locations.

[sduchesneau]

work is on the way: streamingfast/merger#3

doing some tests, awaiting for for PR review, then it should be good to go!

[sduchesneau]

This branch https://github.com/dfuse-io/dfuse-eosio/tree/feature/dedupe-suffixed-oneblockfiles contains the feature.
Works locally.
Mindreader needs new flag --mindreader-oneblock-suffix with its specific suffix, different from the other mindreaders.