`retry` -> `retry_until`

[tony]

libtmux's while-loop based retry() doesn't work. Move to call to poll for a truthy response.

Related:

• libtmux: retry(seconds=...) broke libtmux#368, Modify retry() to take a callable and enforce the timeout libtmux#372
• Layout is broken with 1.9.2 #704 (comment)
• test_workspacebuilder.py won't finish - no error message #620 (via test_workspacebuilder.py won't finish - no error message #620)