[NOMERGE] op-conductor: add configurable raft-mdb backend

[BrianBland]

Adds an optional raft-mdb backend for op-conductor. This is actually the default recommended backend for hashicorp/raft, and is much more performant than bbolt, but requires CGO.

Changes some default raft parameters:

• RAFT_SNAPSHOT_INTERVAL: 120s -> 1s
• RAFT_SNAPSHOT_THRESHOLD: 8192 -> 48
• RAFT_TRAILING_LOGS: 10240 -> 32

These new defaults are better aligned with the use of the conductor fsm logs, which are used to simply record the latest block payload. This can probably be further lowered in a later update.

Adds more metrics to op-conductor to better inspect how time is spent during the payload commit process:

• commit_marshal_duration_seconds
• commit_raft_apply_duration_seconds
• commit_payload_size_bytes
• fsm_apply_duration_seconds
• raft_log_store_duration_seconds

[cb-heimdall]

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