Feature/vector prepend prepop frames

[benjamg]

Fix of the eintr issue leaving half read messages in the socket buffer. Support for push/pop front (and corresponding push/pop back that map onto the old stream system)

There is some duplicate code between the steam conversions and the push_front conversions that really should be unified at some point.

[LindleyF]

I'm getting a few compile errors when I try to build this with CONFIG=debug.

include "cassert" is needed in frame.cpp

message.cpp is checking .sent in an assert, when it should be .is_sent()

Please test pull requests using all three build targets before merging in the future.