QSslSocket: respect read buffer's max size (SecureTransport)

1. QSslSocketBackendPrivate::transmit was ignoring 'readBufferMaxSize';
as a result, we can have a user trying to set read buffer's size
to a small value (and more important - reading slowly in a small
chunks from this socket), but SSL itself socket reading 'too fast',
potentially growing its internal buffer to a huge size. This also
results in auto-tests failing - whenever we're trying to limit read
rate in some test.

2. Update qsslsocket auto-test.

Task-number: QTBUG-43388
Task-number: QTBUG-55170
Change-Id: Iedece26df0ac5b3b7cad62cc8c98aedc28e7ca5b
Reviewed-by: Richard J. Moore <rich@kde.org>