Switch Travis to JDK12

Switch to SecureRandom() instead of getStrongInstance() to avoid
blocking hideously.

Closes #16

Closes #15

Issue #15

If we're keeping some bytes in the line buffer waiting for a
CR/LF, the SSLEngine may not have enough room to put a packet's
worth of data in.

Instead of managing a single buffer, keep a channel of them
and emit a complete buffer each read call.

Closes #14

Mixing substrings and byte array offsets is bad...

SynchronizedSet() requires manual synchronisation when iterating
over it. When returning an iterator make a copy of the set first
to avoid issues with users iterating.

If we multiple long lines in succession the start index got
miscalculated and part of the latter lines discarded.

Move message processors into their own package
Update ktor

After allowing multiple names we started ACKing only the preferred
name, even if the server hadn't advertised it.

Closes #13

Closes #11