`Dialer.DialInternalClient` does not check the circuit breaker but
blindly attempts a connection and can succeed, leaving the system in a
state where there is a healthy connection to a node, but the circuit
breaker used for dialing is open. DistSQL checks for connection health
when scheduling processors, but the connection health check does not
examine the breaker. So DistSQL will proceed to schedule a processor on
a node but then be unable to use the connection to that node because
`Dialer.Dial` will return with a `breaker open` error. The code contains
a TODO to reconcile the handling of circuit breakers in the various
`Dialer` methods, but changing the handling is risky in the short
term. As a stop-gap, we reset the breaker after a connection is
successfully opened.

Fixes cockroachdb#29149

Release note: None

Change `DistSQLPlanner.checkNodeHealth` so that it uses
`nodedialer.Dialer.ConnHealth` instead of `rpc.Context.ConnHealth`. The
former is the right method to be calling to check a node's connection
health.

Refactor `DistSQLPlanner.checkNodeHealth` into a `distSQLNodeHealth`
struct. This removed the need for `DistSQLPlannerTestingKnobs`.

Enhance `nodedialer.Dialer.ConnHealth` to mark connections as unhealthy
if the circuit breaker is open. This prevents DistSQL from planning
processors on such nodes.

Release note: None