This commit enables client-side TLS contexts re-use for zone transfers
over TLS. That, in turn, makes it possible to use the internal session
cache associated with the contexts, allowing the TLS connections to be
established faster and requiring fewer resources by not going through
the full TLS handshake procedure.

Previously that would recreate the context on every connection, making
TLS session resumption impossible.

Also, this change lays down a foundation for Strict TLS (when the
client validates a server certificate), as the TLS context cache can
be extended to store additional data required for validation (like
intermediates CA chain).