RFC: follower reads

Follower reads are consistent reads at historical timestamps from follower
replicas. They make the non-leader replicas in a range suitable sources for
historical reads.

The key enabling technology is the propagation of a **closed timestamp
heartbeats** from the range leaseholder to followers. The closed timestamp
heartbeat (CT heartbeat) is more than just a timestamp. It is a set of
conditions, that if met, guarantee that a follower replica has all state
necessary to satisfy reads at or before the CT.

Consistent historical reads are useful for analytics queries and in particular
allow such queries to be carried out more efficiently and, with appropriate
configuration, away from foreground traffic. But historical reads are also key
to a proposal for [reference-like
tables](#26301) aimed at cutting
down on foreign key check latencies particularly in geo-distributed clusters;
they help recover a reasonably recent consistent snapshot of a cluster after a
loss of quorum; and they are one of the ingredients for [Change Data
Capture](#25229).

Release note: None