Support out-of-order multiframe transfers

[lydia-at-amazon]

Libudpard needs to be able to support reassembling out-of-order packets that are part of a multiframe transfer

[pavel-kirienko]

https://matrix.to/#/!iIoEluFjwtvXWxhOJg:matrix.org/$ahlEFUemvSgNtW7e-8-nj0PuDXEq9GpM--mgP9I-aF8?via=matrix.org&via=lenfesty.ca&via=stratum0.org

/// Consider the following examples, where A and B denote distinct transfers of three frames each:
///
///     A0 A1 A2 B0 B1 B2  -- two transfers without OOO frames; both accepted.
///     A2 A0 A1 B0 B2 B1  -- two transfers with OOO frames; both accepted.
///     A0 A1 B0 A2 B1 B2  -- two transfers with interleaved frames; both accepted (this is why we need 2 buffers).
///
/// It is assumed that interleaved frames spanning more than two transfers are not possible.