core: gatt: Implement GattCharacteristic1.StartNotify

This patch implements the StartNotify method of
org.bluez.GattCharacteristic1. Each call to StartNotify assigns a
session to the D-Bus sender which internally registers a unique
notify handler with the bt_gatt_client. Each received notification
or indication causes a PropertiesChanged signal to be emitted for the
"Value" property.

The notify handler gets automatically unregistered when sender
disconnects from D-Bus.
core: gatt: Implement GattCharacteristic1.StopNotify

This patch implements the StopNotify method of
org.bluez.GattCharacteristic1.
core: gatt: Expose charac. extended properties.

This patch reads the extended properties for characteristics that have
the necessary descriptor. Updating the "Characteristics" property is
delayed for services until the extended properties have been read for
all of their characteristics that have them.
shared/gatt-client: Expose MTU.

This patch adds the bt_gatt_client_get_mtu function. It also includes a
minor fix in which the signature of the "value" argument of write
methods are changed from "uint8_t *" to "const uint8_t *".
core: gatt: Implement GattCharacteristic1.WriteValue.

This patch implements the WriteValue method of
org.bluez.GattCharacteristic1. The GATT write procedure that should be
used is decided based on characteristic properties.
core: gatt: Don't always issue read-long for ReadValue

A remote GATT server may not support the "long write" procedure for all
of its attributes (or not at all). This patch revises the implementation
of GattCharacteristic1.ReadValue and GattDescriptor1.ReadValue, so that
instead of always initiating a "long write" (i.e. ATT Read Blob Req),
they first send out a regular read request and then proceed to a long
write only if the value fills the current MTU and may potentially have
more bytes waiting.
core: gatt: Implement GattDescriptor1.WriteValue.

This patch implements the WriteValue method of org.bluez.GattDescriptor1
core: gatt: Reference count chrcs and descs.

Many of the GATT D-Bus API methods are asynchronous and their callbacks
depend on characteristic and descriptor instances to be alive when they
execute. This patch makes characteristics and descriptors reference
counted so that an interim disconnect or "Service Changed" won't
immediately free up those instances.
core: gatt: Handle Service Changed.

This patch adds handling for Service Changed events. All exported
objects that match attributes within the changed handle range are
unregistered and new ones get exported based on the newly discovered
services.
DO NOT SUBMIT: att_debug