fix: make sure atomic 64bit fields are 64bit aligned

[Stebalien]

Otherwise, this won't work on 32bit ARM.

See https://golang.org/pkg/sync/atomic/#pkg-note-BUG

[Stebalien]

Or we can just fix #5795.

[magik6k]

Yeah, not sure how much we save by not using a RWMutex, it seems like the cleaner solution

We should also review other uses of atomics in the codebase (maybe check if there are any linters which could detect this)

[ribasushi]

@Stebalien afaik filecoin-ffi has a hard-dependency on 64bit instruction sets, so this is moot in the context of lotus

[Stebalien]

TL;DR: There's no reason not to fix this. While we never plan on supporting 32bit platforms, someone else might come along and do that.

Alternatively, someone might try to create a proof-less "lotus lite", extract this code into a separate package, fork, etc.

[yhio]

Yeah, not sure how much we save by not using a RWMutex, it seems like the cleaner solution

We should also review other uses of atomics in the codebase (maybe check if there are any linters which could detect this)

There are many uses of atomics in the codebase.