Switch Travis to JDK12

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

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

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

Fix regression with capability negotiation

To preserve the order of lines we offer them into a channel
without suspending. If we launch a new coroutine there's no
guarantee they'll execute in order, even if the method they
call is fair.