feat(exchange): Add pre-allocating the buffer for reading in HeaderCodec

[fl0rek]

closes #43

Read up to first 10 bytes (max length of the length delimiter varint) in a loop until prost::decode_length_delimiter is able to figure out message length. Then, when reading, preallocate the buffer and read the exact number of bytes needed.

[oblique]

read_response should implement the same mechanism. I suggest implementing a generic read_raw_message that returns the raw bytes and then use it in both.

[zvolin]

looks good.

Recently we've been discussing the proptest as guys in sovereign seems using this alot and this looks like a perfect use for it. We could add fuzz tests for the codec but I leave it up to you guys, @fl0rek @oblique

[oblique]

lgtm