RFC: SELECT FOR UPDATE entering final comment period

The SELECT FOR UPDATE RFC is entering final comment period. Please leave comments on the PR!

For easy reference, the RFC summary is reproduced below:

Support the SELECT ... FOR UPDATE SQL syntax, which locks rows returned by the SELECT statement. Several potential customers have asked for this feature, and it would also get us closer to feature parity with Postgres. The easiest way to implement this in CockroachDB would be by setting row-level “dummy” intents, but another option would be to set span-level locks.