core/rawdb, eth/downloader: align 64bit atomic fields

[karalabe]

The freezer and the downloader contain a few atomic uint64 fields. Fun fact:

On ARM, x86-32, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically. The first word in a variable or in an allocated struct, array, or slice can be relied upon to be 64-bit aligned. https://golang.org/pkg/sync/atomic/#pkg-note-BUG

This PR moves the atomics in the freezer and downloader to the head of the struct to ensure 64bit alignment.

Fixes #19590.