tls: force readable/writable to `true`

These are an old and deprecated properties that was used by previous
stream implementation, and are still in use in some user-land modules.

Prior to this commit, they were read from the underlying socket, which
may be non-readable/non-writable while connecting or while staying
uninitialized.

Force set them to `true`, just to make sure that there will be no
inconsistency.

fix #7152
tls: do not call SNICallback unless present

When asynchronously parsing ClientHello for session resumption -
SNICallback may not be set. Check if it is present before invoking
it.

fix #7010