kv: add --visibility committed, rudb showing a commit before it is durable, for the freshness runs

[tamnd]

Part of tamnd/rudb#1390 (W7), the freshness measure under visibility = committed and visibility = durable.

The freshness line of the CH-benCHmark runs only ever measured rudb at its default, durable, since the driver never set visibility. W7 asks for both.

• --visibility durable|committed, durable unless set. Under committed rudb shows a commit's rows to other sessions once the commit is ordered, before it is durable.
• The Backend trait gets visibility. rudb runs SET visibility = '...' right after commit_sync, and the shim passes it through. Every other engine shows a commit once it is durable, so it takes durable and refuses committed with a sentence saying so, the way level refuses a level an engine cannot set.
• The tpcc header line carries visibility= after isolation=.

Tests

The argument test takes both words and refuses a third.