Use a latch to make startup process wake up and replay immediately when

Use a latch to make startup process wake up and replay immediately when
new WAL arrives via streaming replication. This reduces the latency, and
also allows us to use a longer polling interval, which is good for energy
efficiency.

We still need to poll to check for the appearance of a trigger file, but
the interval is now 5 seconds (instead of 100ms), like when waiting for
a new WAL segment to appear in WAL archive.