tls: remove unneeded atomic load

TlsSession state holder 'state_' is loaded twice
when we check for its modification in the TlsSession
state machine code.
std::atomic_compare_exchange_strong returns false when
'state_' is not equals to 'old_state' and in this case
'old_state' is changed for the current value of 'state_'.
So reload it right after the returns is not efficient
and may also have an unexpected corner case.

This patch re-uses the value of 'old_state' for next
processing.

Change-Id: Ia29abc0f5834caabf12f1ae528ff11b132bd603c
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>