First phase of TransactionSign refactor [RIPD-480]

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TransactionSign.cpp needs refactoring because it requires similar,
but not identical, functionality between submit, sign,
get_signingaccount, and submit_multisigned.  The original code was
structured only for use by sign and submit.  The code needs to be
broken up so it can be used in a more modular way.

This is a step along that path.

 o The method that was called TransactionSubmitImpl () is split into
   two parts:
    a. TransactionConstructImpl () and
    b. TransactionFormatResultImpl ().
   Neither of these pieces actually submit the transaction, since
   neither sign nor get_signingaccount actually submit a transaction
   for processing.

 o The transaction "sterilization" that was occurring in
   NetworkOPs::SubmitTransactionSync () got sucked into
   TransactionConstructImpl (), since that is part of constructing
   a known valid transaction.

 o TransactionSign was the only caller of submitTransactionSync (),
   so that method has been removed from NetworkOPs.

 o Each of transactionSign, transactionSubmit,
   transactionGetSigningAccount, and transactionSubmitMultisigned are
   responsible for calling the pieces that they need.  So they each
   got bigger.

There is more work to be done here.  But I believe that this commit
is an improvement on where we were.