lnd v0.13.1-beta Release Candidate 2

[Olaoluwa Osuntokun]

Hi y'all,

In our typical fashion, we're planning on quickly following the 0.13 release
with a minor release (0.13.1) that focuses on fixing any possible regression
introduced in the major release, and independent bug fixes as well.

If anyone hasn't updated to 0.13 yet for w/e reason, then it may make sense
to hold off for a few days until 0.13.1 is finalized.

From now on, all the release notes for new releases will be included _within_
the git free in the form of a rolling change log. The set of release notes
for 0.13.1 can be found here:
https://github.com/lightningnetwork/lnd/blob/master/docs/release-notes/release-notes-0.13.1.md.

There're two notable PRs that didn't quite make it into 0.13 that are worth
pointing out.

The first an upgrade to lnd's internal output/transaction aggregation engine
(the sweeper) that finally lets it take advantage of the capabilities
provided by anchor-outputs to set fees for HTLC and force close transactions
based on the looming deadline:
https://github.com/lightningnetwork/lnd/pull/5148. With this PR, rather than
using a static confirmation target, we'll instead set our confirmation
target based on the upcoming HTLC (or CSV) deadlines on the commitment
transaction. This change allows lnd to more effectively manage the time
sensitive redemption period when a multi-hop HTLC goes to chain: each time
we go to sweep/resolve an output, we tell the sweeper the deadline that the
output needs to be confirmed by, with the sweeper selecting an confirmation
target appropriately. The next step in this saga is to implement a _dynamic
bumping_ algorithm that slowly increases the fee with each block as we
approach the deadline in order to react to changes in the mempool or block
production rate.

The second is a long-time bug fix that at times has impacted the UX of
wallets that to JIT (just in time) transaction opening when a new user
attempts to receive a payment:
https://github.com/lightningnetwork/lnd/pull/5332. Before this bug fix, lnd
wouldn't properly apply a channel update sent by a _private channel_ (one
advertised in the hop hints), causing common JIT channel schemes to need to
work around this quirk.

Users or wallets the run neutrino nodes will also want to update, as an
optimization inadvertently introduced a regression that would cause an
internal block height to drift over time, causing all payments to fail as
the block height used was too far in the past:
https://github.com/lightningnetwork/lnd/pull/5404.

A regression in the _existing_ keysend API has also been fixed, which was
triggered by the consolidation of the AMP and keysend logic to unify the
code paths: https://github.com/lightningnetwork/lnd/pull/5419.

If nothing new pops up, I expect this minor release to be tagged sometime
next week. Thanks to all that have tested this release candidate, and any
prior ones to help us uncover these bugs and regressions.

-- Laolu