Bluetooth: Add support to get connection information

This patch adds support for Get Connection Information mgmt command
which can be used to query for information about connection, i.e. RSSI
and local TX power level.

In general values cached in hci_conn are returned as long as they are
considered valid, i.e. do not exceed age limit set in hdev. This limit
is calculated as random value between min/max values to avoid client
trying to guess when to poll for updated information.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@tieto.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>