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.
Well, it's hard to say because it's a mess trough out the code base anyway. But a common pattern is: first public than private. First methods than member variables. It's up to you how far you want to recurse into cleaning up. But please don't mix further (if any) cleanup with this commit :)