store: support memoryview values additionally to bytes

[ThomasWaldmann]

Fixes #200.

Store.store(name, value) and backend.store(name, value) now accept a
memoryview of bytes additionally to bytes, so callers can avoid copying:

buffer = bytearray(...)  # e.g. a big buffer, filled with multiple objects
store.store("data/0123456789abcdef", memoryview(buffer)[offset : offset + size])

Implementation

backends/_base.py gets the StoreValue = bytes | memoryview type alias and two helpers:

• validate_value(value): casts a memoryview to a 1-dimensional view of bytes, so
  len(value) always is the number of bytes (relevant for quota tracking and the
  volume statistics, e.g. if the caller has a view with itemsize > 1). A
  non-contiguous memoryview is rejected with a ValueError, because the value must
  be stored as one consecutive sequence of bytes.
• to_bytes(value): for the backends that can not deal with a memoryview.

Per backend (checked against what the used libraries actually do):

backend	memoryview	why
posixfs	no copy	the tempfile write accepts it
rest	no copy	requests sends it as request body, with a correct Content-Length
rclone	no copy	requests puts it into the multipart body
sftp	copied to bytes	paramiko's default file is unbuffered, so write() ends up in util.asbytes(), which raises "Unknown type for encoding" for a memoryview
s3	copied to bytes	botocore rejects it: ParamValidationError: Invalid type for parameter Body

Store.store normalizes the value once, so the volume statistics and the
write-through/mirror cache path are correct, too.

Tests

• test_store_memoryview for all configured backends: a slice of a bigger buffer,
  a view with itemsize 4, an empty view, and the rejection of a non-contiguous view.
• Store level: also checks the item size, the stats volume and that modifying the
  caller's buffer afterwards does not change the stored item.
• Also covered: the write-through cache path, posixfs quota accounting and a real
  HTTP round-trip via the REST server (which also validates the
  X-Content-hash-sha256 header computed from the memoryview).
• sftp and s3 can not be tested here without a server, so there are tests with a
  fake client asserting that paramiko / boto3 get a bytes object.

Docs: "Values" section in docs/store.rst (incl. which backends copy), notes for
the sftp/s3 backends, README and CHANGES.