Merge new TCP BTL code to v2.x

[jsquyres]

#1491 fixed some TCP BTL MPI_THREAD_MULTIPLE issues. After letting it soak on master for a little bit, we need to PR it over to the v2.x branch.

Commits to include:

• 32277db
• 9206249
• f69eba1
• d016581
• f70c5c4
• e852d85
• 7eca2f9

[hppritcha]

There may be some other commits that need to be picked up.

[jsquyres]

This is now done.