Use SendMessage() call

OT still has some remnants of a SSL connection system that was removed in favor
of ZeroMQ. The OTServerConnection class does a lot of stuff that may have made
sense earlier but can be removed and simplified now.

This is the first step of a larger cleanup.

The old code had this repeated pattern

```cpp
m_pClient->SetFocusToServerAndNym(*pServer, *pNym, m_pTransportCallback);
m_pClient->ProcessMessageOut(theMessage);
return m_pClient->CalcReturnVal(lRequestNumber);
```

and a variant that does not have last call to CalcReturnVal()

I replaced these two variants two atomic methods

* `SendMessage(OTServerContract*, OTPseudonym*, OTMessage&, lRequestNumber)`
* `SendMessage(OTServerContract*, OTPseudonym*, OTmessage&)`

The other place where SetFocusToServerAndNym is called now is in opentxs-script
which I will do separately.
@OttoAllmendinger 

Since these methods are only used inside this class they should be declared `private:`.