Increase the default `liquidity_offset_half_life` to six hours

Increase the default `liquidity_offset_half_life` to six hours

Even at relatively high payment volumes, decaying knowledge of each
individual channel every hour causes aggressive retrying of
channels as we quickly forget the state of a channel. Even with the
historical tracker, this isn't fully remedied, as we'll track the
history bounds with the decayed value.

Instead, we decay every six hours here, reducing how often we'll
retry a channel due to decay.

In addition to this, the decay likely needs to be substantially
more linear, as tracked in #1752.