64-bit align replication counters

[briankassouf]

fixes #343

[hanshasselberg]

I didn't know why this is fixing the issue and reading https://go101.org/article/memory-layout.html helped:

The ways are described as the first (64-bit) word in a variable or in an allocated struct, array, or slice can be relied upon to be 64-bit aligned. What does the word allocated mean? We can think an allocated value as a declared variable, a value returned the built-in make function, or the value referenced by a value returned by the built-in new function. If a slice value derives from an allocated array and the first element of the slice is the first element of the array, then the slice value can also be viewed as an allocated value.

The description of which 64-bit words can be relied upon to be 64-bit aligned on 32-bit architectures is some conservative. There are more 64-bit words which can be relied upon to be 8-byte aligned. In fact, if the first element of an array or slice which element type is a 64-bit word type can be relied upon to be 64-bit aligned, then all elements in the array/slice can also be accessed atomically. It is just some subtle and verbose to make a simple clear description to include all the 64-bit words can be relied upon to be 64-bit aligned on 32-bit architectures, so the official documentation just makes a conservative description.

[hanshasselberg]