#23 -- CLA ok -- re-author to me+google@tomthorogood.co.uk

In particular move it to after cipher suite negotiation and after
HelloRetryRequest check.

There should be no data in the Handshake buffer on encryption state
changes (including implicit 1.3 transitions). Checking that also blocks
all Handshake messages fragmented across CCS.

BoGo: PartialClientFinishedWithClientHello

BoGo: Resume-Server-PSKBinderFirstExtension
BoGo: Resume-Server-ExtraPSKBinder
BoGo: Resume-Server-ExtraIdentityNoBinder
BoGo: Renegotiate-Server-Forbidden
BoGo: NoNullCompression
BoGo: TrailingMessageData-*

ConfirmHandshake should block on a Read until the handshakeConfirmed
state is reached, but past that it shouldn't.

Phase should only be accessed under in.Mutex. Handshake and all Read
operations obtain that lock. However, many functions checking for
handshakeRunning only obtain handshakeMutex: reintroduce
handshakeCompleted for them. ConnectionState and Close check for
handshakeConfirmed, introduce an atomic flag for them.

Fixes NCC-2016-002