Handle reload config file request in LR monitor (#8732)

## Problem

Logical replication BGW checking replication lag is not reloading config

## Summary of changes

Add handling of reload config request

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pgxn/neon/neon.c

@@ -192,6 +192,13 @@ LogicalSlotsMonitorMain(Datum main_arg)
 	{
 		XLogRecPtr	cutoff_lsn;
 
+		/* In case of a SIGHUP, just reload the configuration. */
+		if (ConfigReloadPending)
+		{
+			ConfigReloadPending = false;
+			ProcessConfigFile(PGC_SIGHUP);
+		}
+
 		/*
 		 * If there are too many .snap files, just drop all logical slots to
 		 * prevent aux files bloat.