* Document AppendVec
* Remove the almost-duplicate state in AppendVec
AppendVec was maintaining two offsets, `current_len` and
`append_offset`. Despite the different looking names, the two
values have the same meaning, but were updated at slightly different
times. When appending a batch of accounts, `current_len` updates would
be immediately available to other threads after each append, whereas
`append_offset` would only be updated after its mutex was unlocked.
`append_offset` is redundant. By removing it, we eliminate potential
bugs and no longer need to suppress clippy warnings.
* Remove get_mut() from AppendVec design
Only the offset into the AppendVec memory is thread-safe. The memory
itself is only threadsafe because it is append-only and is otherwise
unprotected. Adding get_mut() would only be safe if the memory was
protected by a ReadWrite lock.
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