Calculate the padding required on `ChannelLiquidity` dynamically

Calculate the padding required on `ChannelLiquidity` dynamically

We expect `ChannelLiquidity` to be exactly three cache lines to
ensure the first bytes we need are all one one cache line, but in
practice its a bit more ideal for `ChannelLiquidity`s to always
start on an even cache line as x86 CPUs will often load the
neighboring cache line automatically.

Further, it looks like some versions of `rustc` on some platforms
don't pack `ChannelLiquidity` as well (in #3415) and the next
commit is going to push us over three cache lines anyway.

Instead, here we calculate out the proper padding for
`ChannelLiquidity` to make it align to four 64-byte cache lines.

Should fix #3415.