Added scan implementation to module api.

The implementation expose the following new functions:
1. RedisModule_CursorCreate - allow to create a new cursor object for
keys scanning
2. RedisModule_CursorRestart - restart an existing cursor to restart the
scan
3. RedisModule_CursorDestroy - destroy an existing cursor
4. RedisModule_Scan - scan keys

The RedisModule_Scan function gets a cursor object, a callback and void*
(used as user private data).
The callback will be called for each key in the database proving the key
name and the value as RedisModuleKey.

runtest-moduleapi

@@ -22,4 +22,5 @@ $TCLSH tests/test_helper.tcl \
 --single unit/moduleapi/hooks \
 --single unit/moduleapi/misc \
 --single unit/moduleapi/blockonkeys \
+--single unit/moduleapi/scan \
 "${@}"