core/lb: use SessionID as map index

[jailuthra]

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

Use SessionID instead of ManifestID to index the loadbalancer sessions.

Specific updates (required)

• a3862f1 Use SessionID as map index

How did you test each of these updates (required)

• Run the lb unit tests
• Verify that SessionId is now used within the log lines on a local O

I0305 01:40:55.807153   31849 orchestrator.go:474] Submitted segment to transcode loop manifestID=movmovv sessionID=a7a6b2b9 seqNo=1
I0305 01:40:55.807386   31849 lb.go:54] LB: Using existing transcode session for a7a6b2b9_0

Does this pull request close any open issues?

Fixes #1748

Checklist:

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

[yondonfu]

LGTM!