On ReadAll, WAL seeks to the end of the last record in the tail. If the tail did not
end with preallocated space, the decoder would report 0 as the last offset and begin
writing at offset 0 of the tail.
Fixes#4903
File lock interface was more verbose than it needed to be while
simultaneously making it difficult to support systems (e.g., Windows)
that only permit locked writes on a single fd holding the lock.
Decoder should return error for any broken block including the
one that only contains the length field. We should change io.EOF
to io.ErrUnexpectedEOF before return the error.
