For example in ZODB FileStorage format reader routines are written
working with io.ReaderAt for the following reasons:

- for loads random-access is required,
- there can be several concurrent loads in flight simultaneously.

At the same time various database iterations (APIs additional to load)
use sequential access pattern and can be served by the same record
reading routines. However with them we cannot use e.g. bufio.Reader
because it works with plain io.Reader, not io.ReaderAt.

Here comes SeqReaderAt: it adds a buffer, by default 2·4K, on top of
original io.Reader, automatically detects direction of sequential
access which can be forward, backward, or interleaved forward-backward
patterns, and buffers data accordingly to avoid many syscalls e.g. in
os.File case.