[Tests] Fix flaky tests and memory leak related to ContainerAllocator tests

[cameronlee314]

Issues:

1. TestContainerAllocatorWithHostAffinity.testExpiredRequestsAreCancelled fails transiently due to seeing a value of 4 for cancelledRequests.size() instead of 3.
2. samza-core tests take a lot of heap to run (3GB), but it seems like they shouldn't need that much.

Changes:

1. Change the cluster-manager.container.request.timeout.ms to be 500ms instead of 3ms in TestContainerAllocatorWithHostAffinity in order to prevent the test from cancelling requests too quickly and causing an extra cancellation to happen before the validation is done.
2. Stop the ContainerAllocator and ContainerPlacementManager objects in tests so that their corresponding threads can exit. Before, I believe the threads just kept running, and that may have eaten up memory.
3. Reduce the heap needed for samza-core tests.

Tests: Ran CI build multiple times

API changes: N/A

[Sanil15]

Thank for the Fix