Split client message reading and writing functions

The new state machine code will split up the reading and writing of
hanshake messages into discrete phases. In order to facilitate that the
existing "get" type functions will be split into two halves: one to get
the message and one to process it. The "send" type functions will also have
all work relating to constructing the message split out into a separate
function just for that. For some functions there will also be separate
pre and post "work" phases to prepare or update state.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>