Cancellation semantics and pooler interaction (§7)

[arkstack-dev]

§7 maps cancellation to STOP_SENDING with PG_CANCEL. The race resolution (no-op if no query is running) is in §7 paragraph 2.

For pooler authors and people who have written cancel logic in drivers:

• Does removing BackendKeyData break any deployment topology you rely on?
• For session-pooled topologies (PgBouncer session mode), the mapping is
  straightforward. For transaction or statement pooling, what semantics
  do you want for cancellation that arrives between transactions?