[Minor][Core] fix comments in MapOutputTracker

Using driver and executor in the comments of ```MapOutputTracker``` is more clear.

Author: wangfei <wangfei1@huawei.com>

Closes #3700 from scwf/commentFix and squashes the following commits:

aa68524 [wangfei] master and worker should be driver and executor

(cherry picked from commit 5c24759ddc25cbafbedbaafbf053d38015a7774e)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>