Unable to enable TLS if using v2 protocol

[sijie]

Descriptions of the changes in this PR:

Motivation

TLS is enabled using startTLS but the startTLS
is a v3 protobuf request. So if the client is
configured to use v2 protocol, the client is not
able to decode startTLSResponse.

Modifications

This change is to improve the v2 protocol response handling for
handling startTLS response.

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[eolivelli]

LGTM