server,core: Improve in-flight segment handling

[jailuthra]

What does this pull request do? Explain your changes. (required)

Fix the bugs in, and clean up the logic of the recently introduced flexible segment-in-flight feature between the B & O.

Specific updates (required)

• Remove segments from the in-flight queue only after downloading
• Add goroutines in the test to check for race conditions around selectSession() and completeSession()
• Simplify completeSession logic, and always make sure bsm.lastSess is set to the updated session (with the updated SegsInFlight queue)
• Increase channel buffer count of LB transcoder to match that of the O set previously

How did you test each of these updates (required)

• Manual testing with simultaneous curl calls
• Unit tests with stubbed sessions

Does this pull request close any open issues?

Fixes #1742 and Fixes #1743

Checklist:

• README and other documentation updated
• Node runs in OSX and devenv
• All tests in ./test.sh pass

[iameli]

LGTM!

dismissed per yondon's instructions