This commit provides support for 2-level multi-signing of
transactions. The ability is usually compiled out, since other
aspects of multi-signing are not yet complete.
Here are the missing parts:
o Full support for Tickets in transactions.
o Variable fees based on the number of signers,
o Multiple SignerLists with access control flags on accounts,
o Enable / disable operations based on access control flags,
o Enable / disable all of multi-signing based on an amendment,
o Integration tests, and
o Documentation.
Add support for the SignerListSet transaction as a step toward
multi-sign support.
As part of the SignerListSet implementation, add InnerObjectFormat
templates (similar to TxFormats and LedgerFormats) and enforce them
in STObject, STArray, and STParsedJSON.
