Do not handle lfc_change_limit in processes not haing PGPROC structure (#5332)

## Problem

See https://neondb.slack.com/archives/C05L7D1JAUS/p1693775881474019

## Summary of changes

Do not perform local file cache resizing in processes having no PGPROC

## 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/file_cache.c

@@ -222,8 +222,9 @@ lfc_change_limit_hook(int newval, void *extra)
 	/*
 	 * Stats collector detach shared memory, so we should not try to access shared memory here.
 	 * Parallel workers first assign default value (0), so not perform truncation in parallel workers.
+	 * The Postmaster can handle SIGHUP and it has access to shared memory (UsedShmemSegAddr != NULL), but has no PGPROC.
 	 */
-	if (!lfc_ctl || !UsedShmemSegAddr || IsParallelWorker())
+	if (!lfc_ctl || !MyProc || !UsedShmemSegAddr || IsParallelWorker())
 		return;
 
 	/* Open cache file if not done yet */

test_runner/regress/test_lfc_resize.py

@@ -0,0 +1,44 @@
+import threading
+import time
+
+import pytest
+from fixtures.log_helper import log
+from fixtures.neon_fixtures import NeonEnv, PgBin
+
+
+#
+# Test branching, when a transaction is in prepared state
+#
+@pytest.mark.timeout(600)
+def test_lfc_resize(neon_simple_env: NeonEnv, pg_bin: PgBin):
+    env = neon_simple_env
+    env.neon_cli.create_branch("test_lfc_resize", "empty")
+    endpoint = env.endpoints.create_start(
+        "test_lfc_resize",
+        config_lines=[
+            "neon.file_cache_path='file.cache'",
+            "neon.max_file_cache_size=1GB",
+            "neon.file_cache_size_limit=1GB",
+        ],
+    )
+    n_resize = 10
+    scale = 10
+    log.info("postgres is running on 'test_lfc_resize' branch")
+
+    def run_pgbench(connstr: str):
+        log.info(f"Start a pgbench workload on pg {connstr}")
+        pg_bin.run_capture(["pgbench", "-i", f"-s{scale}", connstr])
+        pg_bin.run_capture(["pgbench", "-c4", f"-T{n_resize}", "-Mprepared", connstr])
+
+    thread = threading.Thread(target=run_pgbench, args=(endpoint.connstr(),), daemon=True)
+    thread.start()
+
+    conn = endpoint.connect()
+    cur = conn.cursor()
+
+    for i in range(n_resize):
+        cur.execute(f"alter system set neon.file_cache_size_limit='{i*10}MB'")
+        cur.execute("select pg_reload_conf()")
+        time.sleep(1)
+
+    thread.join()