Backport: Add merge blocklog feature

[vladtr]

Allow eosio-blocklog to merge split block log files.

Please refer to these 2 PR's:
EOSIO/eos#10078
EOSIO/eos#10084

[oschwaldp-oci]

Blocklog refactor related. May not plan to backport this due to differences with blocklog changes, but instead work it when backlog path forward decided.