wireguard: add async mode for decryption packets

wireguard: add async mode for decryption packets

Originally wireguard doesn't support async mode for decryption packets.

This patch add async mode for decryption in wireguard.
In addition, it contains some performance improvement such as
prefetching packet header and reducing the number of current time
function calls.

Type: improvement

Signed-off-by: Gabriel Oginski <gabrielx.oginski@intel.com>
Change-Id: Ieba6ae0078f3ff140c05b517891afb57232b3b7d