Replaced 'wait' blpop method with 'echo' healthcheck to avoid slave read-only failures.

Detect and warn when minimal required node managers is less than majority.
Fix slave-serve-stale-data tests.

Resolves issues https://github.com/ryanlecompte/redis_failover/issues/63 and  https://github.com/ryanlecompte/redis_failover/issues/65