Show sizes of FETCH queries as constants in pg_stat_statements

Prior to this patch, every FETCH call would generate a unique queryId
with a different size specified.  Depending on the workloads, this could
lead to a significant bloat in pg_stat_statements, as repeatedly calling
a specific cursor would result in a new queryId each time.  For example,
FETCH 1 c1; and FETCH 2 c1; would produce different queryIds.

This patch improves the situation by normalizing the fetch size, so as
semantically similar statements generate the same queryId.  As a result,
statements like the below, which differ syntactically but have the same
effect, will now share a single queryId:
FETCH FROM c1
FETCH NEXT c1
FETCH 1 c1

In order to do a normalization based on the keyword used in FETCH,
FetchStmt is tweaked with a new FetchDirectionKeywords.  This matters
for "howMany", which could be set to a negative value depending on the
direction, and we want to normalize the queries with enough information
about the direction keywords provided, including RELATIVE, ABSOLUTE or
all the ALL variants.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0tA6LbHCg2qSS+KuM850BZC_+ZgHV7Ug6BXw22TNyF+MA@mail.gmail.com

contrib/pg_stat_statements/sql/cursors.sql

@@ -28,3 +28,46 @@ COMMIT;
 
 SELECT calls, rows, query FROM pg_stat_statements ORDER BY query COLLATE "C";
 SELECT pg_stat_statements_reset() IS NOT NULL AS t;
+
+-- Normalization of FETCH statements
+BEGIN;
+DECLARE pgss_cursor CURSOR FOR SELECT FROM generate_series(1, 10);
+-- implicit directions
+FETCH pgss_cursor;
+FETCH 1 pgss_cursor;
+FETCH 2 pgss_cursor;
+FETCH -1 pgss_cursor;
+-- explicit NEXT
+FETCH NEXT pgss_cursor;
+-- explicit PRIOR
+FETCH PRIOR pgss_cursor;
+-- explicit FIRST
+FETCH FIRST pgss_cursor;
+-- explicit LAST
+FETCH LAST pgss_cursor;
+-- explicit ABSOLUTE
+FETCH ABSOLUTE 1 pgss_cursor;
+FETCH ABSOLUTE 2 pgss_cursor;
+FETCH ABSOLUTE -1 pgss_cursor;
+-- explicit RELATIVE
+FETCH RELATIVE 1 pgss_cursor;
+FETCH RELATIVE 2 pgss_cursor;
+FETCH RELATIVE -1 pgss_cursor;
+-- explicit FORWARD
+FETCH ALL pgss_cursor;
+-- explicit FORWARD ALL
+FETCH FORWARD ALL pgss_cursor;
+-- explicit FETCH FORWARD
+FETCH FORWARD pgss_cursor;
+FETCH FORWARD 1 pgss_cursor;
+FETCH FORWARD 2 pgss_cursor;
+FETCH FORWARD -1 pgss_cursor;
+-- explicit FETCH BACKWARD
+FETCH BACKWARD pgss_cursor;
+FETCH BACKWARD 1 pgss_cursor;
+FETCH BACKWARD 2 pgss_cursor;
+FETCH BACKWARD -1 pgss_cursor;
+-- explicit BACKWARD ALL
+FETCH BACKWARD ALL pgss_cursor;
+COMMIT;
+SELECT calls, query FROM pg_stat_statements ORDER BY query COLLATE "C";