Set nonblocking mode on the right socket.

After a new connection has been accepeted, mark this new socket as
non-blocking, instead of remarking the listening socket once again.
Drop unneeded asserts.

These two asserts serve no purpose and break things. It's okay if a
socket is closed prematurely without sending all the data to its peer.
Tunable crashlog and some bugfixes.

Crashlog size is now tunable, size = 0 disables it completely. Also
fixed some minor bugs in the slave & engine code.
Changelog bump.
Merge branch 'master' of github.com:cocaine/cocaine-core