Allow to specify replication delay by time.

Allow to specify replication delay by time.

delay_threshold specifies replication delay upper limit in bytes. Add
similar parameter called delay_threshold_by_time so that the limit can
be specified by time (seconds). The new parameter is effective if it is
greater than 0 and track_commit_timestamp (available in >> PostgreSQL
9.5 or after) is enabled. In this case "show pool_ndoes" and
pcp_node_info display the replication delay in seconds. If the
parameter is set to 0 or track_commit_timestamp is not enabled,
delay_threshold_by_time is ignored and falls back to delay_threshold
mode.

For this purpose new member standby_delay_by_time is added to shared
memory data Backendinfo to distinguish whether replication delay is
measured in byte (standby_delay_by_time == false) or seconds
(standby_delay_by_time = true). If standby_delay_by_time is true,
standby_delay is measured in second * 1000000, so that the precision
is 6 digits after decimal point.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2021-December/004081.html