Add the ability to disable the covering-index-scan optimization at compile-time,

start-time, or at run-time.  Add test cases to check this configurability.

FossilOrigin-Name: ccb8ecc30c8e6c7760131250297c2e452bbac43b

src/global.c

@@ -133,6 +133,10 @@ const unsigned char sqlite3CtypeMap[256] = {
 # define  SQLITE_USE_URI 0
 #endif
 
+#ifndef SQLITE_ALLOW_COVERING_INDEX_SCAN
+# define SQLITE_ALLOW_COVERING_INDEX_SCAN 1
+#endif
+
 /*
 ** The following singleton contains the global configuration for
 ** the SQLite library.
@@ -142,6 +146,7 @@ SQLITE_WSD struct Sqlite3Config sqlite3Config = {
    1,                         /* bCoreMutex */
    SQLITE_THREADSAFE==1,      /* bFullMutex */
    SQLITE_USE_URI,            /* bOpenUri */
+   SQLITE_ALLOW_COVERING_INDEX_SCAN,   /* bUseCis */
    0x7ffffffe,                /* mxStrlen */
    128,                       /* szLookaside */
    500,                       /* nLookaside */