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/impl/Sign.cpp

@@ -51,4 +51,21 @@ verify (STObject const& st,
                 true);
 }
 
+Serializer
+buildMultiSigningData (STObject const& obj, AccountID const& signingID)
+{
+    Serializer s {startMultiSigningData (obj)};
+    finishMultiSigningData (signingID, s);
+    return s;
+}
+
+Serializer
+startMultiSigningData (STObject const& obj)
+{
+    Serializer s;
+    s.add32 (HashPrefix::txMultiSign);
+    obj.addWithoutSigningFields (s);
+    return s;
+}
+
 } // ripple