Show sizes of FETCH queries as constants in pg_stat_statements

 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