Improve unit testing of ambry-server, ambry-store and ambry-replication.

[vgkholla]

Currently, much of the code in ambry-server, ambry-store and amby-replication is being tested via integration tests in ambry-server. Unit testing coverage has to be improved in these packages so that failures are caught faster, bugs are easy to isolate and debugging is easier.

Doing this will require understanding of each of these components. A starting point would be running the unit tests in each of these packages, checking the coverage, understanding the classes that need more coverage and writing appropriate tests. Aim to cover all lines as well as cases.

[sriramsub]

I don't get what needs to be improved from the description. Without very clear definition of what unit test needs to be written, I don't see this to be a newbie issue.

[vgkholla]

What needs to be improved is unit test coverage since most of the code in these packages is being tested via integration tests and not unit tests.

This is a high level ticket to investigate what unit tests need to be written and write them. My thought process is that it would encourage a newbie to read the code and at the same time contribute something useful. We can always create individual issues for particular tests.

I can remove the newbie tag if you think we need to create individual issues that can be tagged newbie.