Motivation:
There are some use cases when a client may only be willing to read from a channel once
its previous write is finished (eg: serial dispatchers in Finagle). In this case, a
connection with SslHandler installed and ctx.channel().config().isAutoRead() == false
will stall in 100% of cases no matter what order of "channel active", "write", "flush"
events was.
The use case is following (how Finagle serial dispatchers work):
1. Client writeAndFlushes and waits on a write-promise to perform read() once it's satisfied.
2. A write-promise will only be satisfied once SslHandler finishes with handshaking and
sends the unencrypted queued message.
3. The handshaking process itself requires a number of read()s done by a client but the
SslHandler doesn't request them explicitly assuming that either auto-read is enabled
or client requested at least one read() already.
4. At this point a client will stall with NEED_UNWRAP status returned from underlying engine.
Modifiations:
Always request a read() on NEED_UNWRAP returned from engine if
a) it's handshaking and
b) auto read is disabled and
c) it wasn't requested already.
Result:
SslHandler is now completely tolerant of whether or not auto-read is enabled and client
is explicitly reading a channel.
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