#48 Have CTX functions to get info relevant for transfer Callback

[Martin Peylo]

CMP_PKIMESSAGE_http_perform() could be overwritten by means of a callback.

Ensure best practice information hiding by having access functions allowing to get the relevant information without needing to know the internal structure of ctx.

ctx->msgTimeOut
ctx->serverName
ctx->serverPort
ctx->tlsBIO <-- maybe not needed
ctx->proxyName
ctx->proxyPort

others?

Also add a void pointer to the ctx to allow handing other possibly relevant information to the transfer function, e.g. source IP.