We rename `ChanUpdateOptionMaxHtlc` to `ChanUpdateRequiredMaxHtlc`
as with the latest changes it is now required.
Similarly, rename `validateOptionalFields` to
`ValidateChannelUpdateFields`, export it to use it in a later commit.
This commit was previously split into the following parts to ease
review:
- 2d746f68: replace imports
- 4008f0fd: use ecdsa.Signature
- 849e33d1: remove btcec.S256()
- b8f6ebbd: use v2 library correctly
- fa80bca9: bump go modules
This commit changes the WriteElement and WriteElements methods to take a
write buffer instead of io.Writer. The corresponding Encode methods are
changed to use the write buffer.
This commit changes the method WriteMessage to use bytes.Buffer to save
heap allocations. A unit test is added to check the method is
implemented as expected.