This commit removes obsolete comments, dead or no longer useful
code, and workarounds for several issues that were present in older
compilers that we no longer support.
Specifically:
- It improves the transaction metadata handling class, simplifying
its use and making it less error-prone.
- It reduces the footprint of the Serializer class by consolidating
code and leveraging templates.
- It cleanups the ST* class hierarchy, removing dead code, improving
and consolidating code to reduce complexity and code duplication.
- It shores up the handling of currency codes and the conversation
between 160-bit currency codes and their string representation.
- It migrates beast::secure_erase to the ripple namespace and uses
a call to OpenSSL_cleanse instead of the custom implementation.
Payment channels permit off-ledger checkpoints of XRP payments flowing
in a single direction. A channel sequesters the owner's XRP in its own
ledger entry. The owner can authorize the recipient to claim up to a
give balance by giving the receiver a signed message (off-ledger). The
recipient can use this signed message to claim any unpaid balance while
the channel remains open. The owner can top off the line as needed. If
the channel has not paid out all its funds, the owner must wait out a
delay to close the channel to give the recipient a chance to supply any
claims. The recipient can close the channel at any time. Any transaction
that touches the channel after the expiration time will close the
channel. The total amount paid increases monotonically as newer claims
are issued. When the channel is closed any remaining balance is returned
to the owner. Channels are intended to permit intermittent off-ledger
settlement of ILP trust lines as balances get substantial. For
bidirectional channels, a payment channel can be used in each direction.
