DyTask scenario (#406)

Stubbed out scenario class and created unit test.
Add todos and refactor
Add missing header
Updated todo
Correct PMD unused field warning
#369 Fixed with unit test.
#369 Fix after review
#369 Fix after review
#439 tests skipped for MacOS
layout
NginxTest: unbreak unit tests under MacOS with BSD's sed.

Replaces usage of sed by similar perl command.

I also revert a small part of changes from 7321217596f69c2e4b312862a9407bea4f96c8c4
commit where egor@ disabled these tests under MacOS.

Resolve #447 (and also fix #439 properly).
tests skipped
#449 Refectored update config methods
#449 Fix after review
#372 Implementation of txn method
#443 DyReposITCase
#405 Implement memo method.

Added DyMemo and related test cases.
todo: implement update method
#372: Implementation of txn method
Revert "#372 Implementation of txn method"

This reverts commit 83d8c088ac2e94df708d409bbec9ffb5c4f95cc8.