Flush redis only on deploys but not on regular restarts

[alexey-yarmosh]

• on build, write the current commit hash to a file in /dist;
• on app start, every instance reads API_COMMITHASH${HOSTNAME} from redis, and reads the latest commit hash from the file;
• if the values exist and match, don't purge the DB; otherwise purge and write the new commit value to redis;