debugger: remove unused definition `currentSource`

Client's property `currentSource` has no use throughout the project.
tls: fix crash in SNICallback

`tls_wrap.cc` was crashing in an `Unwrap` call, when non
`SecureContext` object was passed to it. Check that the passed object
is a `SecureContext` instance before unwrapping it.

fix #7008
zlib: separate sync/async methods