Dynamically change a udp multicast peer's read buffer

[sergiu128]

Consider the following scenario: (showcased in the test)

• exchange sends data on two multicast groups A and B
• we have 2 udp peers, one for A and one for B which are scheduled to read data
• these 2 udp peers must share the same read buffers. Initially they are both scheduled to read into b1.

If peer A reads something in b1, peer B must not overwrite what has been read by peer A. Instead, peer B must complete its read into whatever buffer peer A now reads into, say b2. That means peer A must change the buffer peer B reads into in its read callback. To summarize:

• peer A and B schedule an async read into b1
• peer A reads and processes what is in b1 (in its async callback)
• peer B did not read yet. It must not read into b1
• peer A schedules itself to read into b2 and also changes peer B to read into b2 through B.SetAsyncReadBuffer (in peer A's read callback)
• by the end of peer A's read, both peer A and peer B will be scheduled to read into b2.

More concretely, for edxm historical feed b1, b2 etc are chunks obtained from a bip buffer.

[avgoustinos-kadis]

if this indeed needs to be on every AsyncRead when having multiple readers, then might be a good idea to introduce a peers manager that provides you with an AsyncRead that already takes care of this

[sergiu128]

agreed! I think this functionality should be in ava:gateway/lib though such that multiple exchanges can use it without worrying about this detail. I'll make it part of this PR