Create a new pager type, PAGER_SORTER, for use in the external merge sort.

Such pagers are always held in memory but do report when they are under
memory pressure by calling pagerStress.

FossilOrigin-Name: c71d73201d950355862dd8d5de142c9673888755

src/btree.h

@@ -61,6 +61,7 @@ int sqlite3BtreeOpen(
 #define BTREE_MEMORY        4  /* This is an in-memory DB */
 #define BTREE_SINGLE        8  /* The file contains at most 1 b-tree */
 #define BTREE_UNORDERED    16  /* Use of a hash implementation is OK */
+#define BTREE_SORTER       32  /* Used as accumulator in external merge sort */
 
 int sqlite3BtreeClose(Btree*);
 int sqlite3BtreeSetCacheSize(Btree*,int);