Simple multisigning (RIPD-182):

With this changeset two-level multisigning is removed from the
codebase and replaced with single-level multisigning.

Additionally, SignerLists in the ledger are prepared for the
possibility of multiple SignerLists per account.  This was done
by adding a defaulted 32-bit SignerListID to each SignerList.
The SignerListIndex calculation incorporates the SignerListID.

There are three known missing elements:

 1. Multisigned transactions should require higher fees than
    regular (single-signed) transaction.  That's not yet
    implemented.

 2. It should be possible to disable the master key on an account
    if that account is multisign enabled (has a signer list).
    That's not yet implemented.

 3. Documentation about multisigning needs to be improved.

Multisigning is still compiled out of the code base.  To enable
multisigning for a stand-alone rippled, change the
RIPPLE_ENABLE_MULTI_SIGN macro (in BeastConfig.h) to "1" and
rebuild.

This commit also addresses:
 o RIPD-912: Remove multisign APIs from STObject, and
 o RIPD-944: Replace common_transactor with jtx at call sites.

src/ripple/protocol/STObject.h

@@ -423,20 +423,6 @@ public:
     uint256 getHash (std::uint32_t prefix) const;
     uint256 getSigningHash (std::uint32_t prefix) const;
 
-    // Break the multi-signing hash computation into 2 parts for optimization.
-    Serializer startMultiSigningData () const;
-    void finishMultiSigningData (
-        AccountID const& signingForID,
-        AccountID const& signingID, Serializer& s) const;
-
-    // VFALCO Get functions are usually simple observers but
-    //        this one performs an expensive construction.
-    //
-    // Get data to compute a complete multi-signature.
-    Serializer getMultiSigningData (
-        AccountID const& signingForID,
-        AccountID const& signingID) const;
-
     const STBase& peekAtIndex (int offset) const
     {
         return v_[offset].get();