[gobwas]

ws,wsutil: do not trust peer-declared payload length for allocation

Payloads are now read through internal/saferio.ReadN(): at most 32 KiB
are allocated before the bytes actually arrive, the buffer grows with
them and ends with an exact capacity.

ws.ReadFrame() and MustReadFrame() are deprecated: the package has no
knob to bound a payload and none is in the RFC; it's now encouraged to
use ReadHeader() and check Header.Length instead.

wsutil.Reader gets MaxMessageSize, the total of all fragments (RFC 6455
section 10.4), which MaxFrameSize alone never bounded. The helpers are
configured with goroutine-safe SetMaxFrameSize(), SetMaxMessageSize(),
SetCheckUTF8() and SetPreallocLimit(), each with a getter, and
DefaultReader() returns the Reader they use. README and the package doc
recommend setting the limits.

Refs #223. Closes #229.