Use smgrexists() instead of access() to enforce uniqueness of generated relfilenumber (#7992)

## Problem

Postgres is using `access()` function in `GetNewRelFileNumber` to check
if assigned relfilenumber is not used for any other relation. This check
will not work in Neon, because we do not have all files in local
storage.

## Summary of changes

Use smgrexists() instead which will check at page server if such
relfilenode is used.

## 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>

test_runner/regress/test_oid_overflow.py

@@ -0,0 +1,45 @@
+from fixtures.log_helper import log
+from fixtures.neon_fixtures import NeonEnvBuilder
+
+
+def test_oid_overflow(neon_env_builder: NeonEnvBuilder):
+    env = neon_env_builder.init_start()
+
+    endpoint = env.endpoints.create_start("main")
+
+    conn = endpoint.connect()
+    cur = conn.cursor()
+
+    cur.execute("CREATE EXTENSION neon_test_utils")
+
+    cur.execute("CREATE TABLE t1(x integer)")
+    cur.execute("INSERT INTO t1 values (1)")
+    cur.execute("CREATE TABLE t2(x integer)")
+    cur.execute("INSERT INTO t2 values (2)")
+
+    cur.execute("SELECT x from t1")
+    assert cur.fetchone() == (1,)
+    cur.execute("SELECT x from t2")
+    assert cur.fetchone() == (2,)
+
+    cur.execute("VACUUM FULL t1")
+    cur.execute("VACUUM FULL t1")
+    cur.execute("vacuum pg_class")
+    cur.execute("SELECT relfilenode FROM pg_class where relname='t1'")
+    oid = cur.fetchall()[0][0]
+    log.info(f"t1.relfilenode={oid}")
+
+    cur.execute("set statement_timeout=0")
+    cur.execute(f"select test_consume_oids({oid-1})")
+    cur.execute("VACUUM FULL t2")
+
+    cur.execute("SELECT relfilenode FROM pg_class where relname='t2'")
+    oid = cur.fetchall()[0][0]
+    log.info(f"t2.relfilenode={oid}")
+
+    cur.execute("SELECT clear_buffer_cache()")
+
+    cur.execute("SELECT x from t1")
+    assert cur.fetchone() == (1,)
+    cur.execute("SELECT x from t2")
+    assert cur.fetchone() == (2,)