Add a new option --enableMajorityReadConcern. It will be enabled by default on CSRS nodes. When enabled, we will fail to start up if the selected storage engine doesn't support snapshots. When disabled, requests with readMajority=majority will fail with a dedicated error.
At some point during the RCs before GA release, we will disable the snapshot thread if --enableMajorityReadConcern is not specified (
SERVER-20825). Until then, we will leave the thread enabled by default until then to allow more burn-in time across more workloads.