harden checks around using --shardSvr, --overrideShardIdentity, and queryableBackupMode

in queryableBackupMode:

• require --overrideShardIdentity if started with --shardsvr
• add a warning if started with --overrideShardIdentity but not --shardsvr

in non-queryableBackupMode:

• fail if started with --overrideShardIdentity
• add a warning if a shardIdentity document exists on disk but not started with --shardsvr
• add a warning if no shardIdentity document exists on disk but started with --shardsvr

additionally:

• use Date_t::max() as the deadline for initializeFromShardIdentity() in all cases
• generally refactor startup and shard aware initialization code for clarity and conciseness