SAMZA-2284: Remove redundant getStreamMetadata calls in SamzaContainer startup sequence.

[shanthoosh]

Problem:

SamzaContainer startup sequence fetches the metadata of same input streams multiple times. Fetching the metadata of a stream entails making a expensive remote call to underlying messaging broker. This redundant fetch-input-stream-metadata API invocations incurred significant delays in the start of actual message processing in the samza job.

Impact:

1. With some samza jobs at LinkedIn, we observed that this fetch-input-stream-metadata loop took around 1.5 hrs to complete.
2. The redundant fetch-input-stream-metadata remote API calls will significantly increase the load on the underlying messaging broker.

Fix:

This patch fixes the unnecessary delay by fetching the metadata of the input streams only once in SamzaContainer(rather than once per SystemStreamPartition per task).

Validation:

1. Fix the existing unit-tests.
2. Verified this change with a sample samza-yarn job in LinkedIn and with a hello-samza job from samza-hello-world examples in OSS.

[shanthoosh]

@prateekm
Please take a look when you get a chance.

[dnishimura]

Looks good overall. Just a minor question.