Revert of Remove global list of SRTP sessions.

Revert of Remove global list of SRTP sessions. (patchset #4 id:60001 of https://codereview.webrtc.org/1416093010/ )

Reason for revert:
Unfortunately this breaks an internal downstream project since we have an ancient libsrtp. Reverting until we can figure out how to update our libsrtp.

Original issue's description:
> Remove global list of SRTP sessions.
> Instead save a reference to the SrtpSession inside the srtp_ctx_t.
>
> BUG=webrtc:5133
>
> Committed: https://crrev.com/9cafd972779ed7b25886ab276e0ede7b7a8b76a1
> Cr-Commit-Position: refs/heads/master@{#10591}

TBR=juberti@google.com,juberti@webrtc.org,jbauch@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5133
Committed: https://crrev.com/cbe9f51cf85a5aeb20a5134dad56cd2b527c098d
Cr-Commit-Position: refs/heads/master@{#10635}

[phoglund, 2015-11-13 14:54:55]

Created Revert of Remove global list of SRTP sessions.

[commit-bot: I haz the power, 2015-11-13 14:55:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1442863003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1442863003/1

[commit-bot: I haz the power, 2015-11-13 14:55:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-13 14:55:27]

Patchset 1 (id:??) landed as
https://crrev.com/cbe9f51cf85a5aeb20a5134dad56cd2b527c098d
Cr-Commit-Position: refs/heads/master@{#10635}