Fix Shutdown behavior for batchprocessor

I added a Shutdown() test that does basic verification of the behavior of the
Shutdown() function. More verifications can be added later.

The test revealed a bug in batchprocessor Shutdown() function which would
hang in Consume* functions after shutdown was called because it was not possible
to send to the channel that the batchprocessor uses.

I will add tests for more components in subsequent PRs. This work is
necessary to ensure Shutdown() works correctly (we will see in future
PRs that we have other bugs that need to be fixed). The test is written
in a generic way that can be used for other components.

Author: tigrannajaryan