PG-254: Removal of pgss_query_hash hash table.

There was no necessity for using a separate hash table to keep track
of queries as all the necessary information is already available in
the pgss_hash table.

The code that moves pending queries' text in hash_query_entry_dealloc
was merged into hash_entry_dealloc.

Couple functions were not necessary anymore, thus were removed:
- hash_create_query_entry
- pgss_store_query_info
- pgss_get_entry (this logic was added directly into pgss_store).

We simplified the logic in pgss_store, it basically works as follows:
1. Create a key (bucketid, queryid, etc...) for the query event.
2. Lookup the key in pgss_hash, if no entry exists for the key, create a
   new one, save the query text into the current query buffer.
3. If jstate == NULL, then update stats counters for the entry.

pg_stat_monitor.h

@@ -380,7 +380,7 @@ void hash_entry_reset(void);
 void hash_query_entryies_reset(void);
 void hash_query_entries();
 void hash_query_entry_dealloc(int new_bucket_id, int old_bucket_id, unsigned char *query_buffer[]);
-bool hash_entry_dealloc(int new_bucket_id, int old_bucket_id);
+void hash_entry_dealloc(int new_bucket_id, int old_bucket_id, unsigned char *query_buffer[]);
 pgssEntry* hash_entry_alloc(pgssSharedState *pgss, pgssHashKey *key, int encoding);
 Size hash_memsize(void);