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/Indexes.cpp

@@ -181,9 +181,14 @@ getRippleStateIndex (AccountID const& a, Issue const& issue)
 uint256
 getSignerListIndex (AccountID const& account)
 {
+    // We are prepared for there to be multiple SignerLists in the future,
+    // but we don't have them yet.  In anticipation of multiple SignerLists
+    // We supply a 32-bit ID to locate the SignerList.  Until we actually
+    // *have* multiple signer lists, we can default that ID to zero.
     return sha512Half(
         std::uint16_t(spaceSignerList),
-        account);
+        account,
+        std::uint32_t (0));  // 0 == default SignerList ID.
 }
 
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