Add a fatal error condition for invalid Dictionary insertion distances

When choosing poor/aggressive values for `table_expire_interval`,
`table_expire_delay`, and/or `table_incremental_step` that tend to
leave tables in state of constant table-expiry-iteration, the underlying
Dictionary is never allowed the chance to complete remapping operations
which re-position entries to more ideal locations (e.g. after
reallocating the table to be able to store more entries).

That situation not only leads to the Dictionary generally having a less
efficient structure, but eventually, the lack of re-positioning may
cause an insertion to calculate the new entry's
distance-from-ideal-position to be a value requiring a full 16-bits or
more (>=65535), but an entry only allows storing 16-bit distance values,
with 65535 being a sentinel value that is supposed to indicate an empty
entry.  Dictionary operations may start misbehaving if that's allowed to
happen.

src/Dict.cc

@@ -1027,8 +1027,14 @@ int Dictionary::LookupIndex(const void* key, int key_size, detail::hash_t hash,
 		*insert_position = i;
 
 	if ( insert_distance )
+		{
 		*insert_distance = i - bucket;
 
+		if ( *insert_distance >= detail::TOO_FAR_TO_REACH )
+			reporter->FatalErrorWithCore("Dictionary (size %d) insertion distance too far: %d",
+			                             Length(), *insert_distance);
+		}
+
 	return -1;
 	}