Connection.readLP not protected against unreasonable sizes

[arnetheduck]

https://github.com/status-im/nim-libp2p/blob/691efaa8a12a4055719fd9fb1b91b46e1944c12a/libp2p/connection.nim#L112

since the remote end sends a size first, it can force the application to try to allocate ridiculous amounts of memory - should require a max length.

[cheatfate]

@arnetheduck but it is covered with check for size < DefaultReadSize.

[arnetheduck]

right, missed it

[sinkingsugar]

Btw this is kinda wrong, the value of DefaultReadSize is not in the spec, and prevents mplex 1MB max size messages too.
See: #74 (comment)
Also chronos seems to fail with big sizes in stream transport somehow status-im/nim-chronos#72