Handshake before reading metadata

[hinerm]

Currently metadata key, value pairs are read during ReadImageInformation without any handshaking to signify the start of the read. This means that, say, starting the JVM with the agentlib flag - which prints a "listening on port xxxx" - destroys the metadata table, which kills the whole process.

So we just need a handshaking step that discards anything in the input stream before reading K,V pairs.