Represent command completion tags as structs

The backend was using strings to represent command tags and doing string
comparisons in multiple places, but that's slow and unhelpful.  Create a
new command list with a supporting structure to use instead; this is
stored in a tag-list-file that can be tailored to specific purposes with
a caller-definable C macro, similar to what we do for WAL resource
managers.  The first first such uses are a new CommandTag enum and a
CommandTagBehavior struct.

Replace numerous occurrences of char *completionTag with a
QueryCompletion struct so that the code no longer stores information
about completed queries in a cstring.  Only at the last moment, in
EndCommand(), does this get converted to a string.

EventTriggerCacheItem no longer holds an array of palloc’d tag strings
in sorted order, but rather just a Bitmapset over the CommandTags.

Author: Mark Dilger, with unsolicited help from Álvaro Herrera
Reviewed-by: John Naylor, Tom Lane
Discussion: https://postgr.es/m/981A9DB4-3F0C-4DA5-88AD-CB9CFF4D6CAD@enterprisedb.com

src/include/commands/createas.h

@@ -22,7 +22,8 @@
 
 
 extern ObjectAddress ExecCreateTableAs(ParseState *pstate, CreateTableAsStmt *stmt,
-									   ParamListInfo params, QueryEnvironment *queryEnv, char *completionTag);
+									   ParamListInfo params, QueryEnvironment *queryEnv,
+									   QueryCompletion *qc);
 
 extern int	GetIntoRelEFlags(IntoClause *intoClause);