Bluetooth: Add support for reusing the same hci_conn for LE links

As most LE devices leave advertising mode when they enter the connected
state, we may want to "pass" that connection to other users.

The first user will be the pairing procedure, the connection is
established without an associated socket, after the pairing is
complete, userspace may want to discover via GATT what services the
newly bonded device has.

If userspace establishes the connection while the timeout still
hasn't expired, the connection will be re-used.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Tested-by: João Paulo Rechi Vita <jprvita@openbossa.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>