Move TaskExecutor to the top of task_executor.h
Move TaskExecutor to the top of task_executor.h

This is the typical convention and puts the most important class in the
header file (its namesake) at the top of the file.

Change-Id: Ie94e0cadf46327e1f34a7c47b956a97923e09ab4
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3223
Tested-by: jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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