MEDIUM: connection: avoid calling handshakes when polling is required

MEDIUM: connection: avoid calling handshakes when polling is required

If a data handler suddenly switches to a handshake mode and detects the
need for polling in either direction, we don't want to loop again through
the handshake handlers because we know we won't be able to do anything.

Similarly, we don't want to call again the data handlers after a loop
through the handshake handlers if polling is required.

No performance change was observed, it might only be observed during
high rate SSL renegociation.