Fix some code style issues on adapter_state_change_cb

Remove extra space and replace tab with space.
Add small check on cmd_enable

In order to enable adapter, we do a call for register_client of GATT
client interface. So we need to check if we have that interface before
call it.
Start support to multiple connections

We replicate any attribute related to a specific connection.
Add connections command

In order to support multiple connections, we need to pass connection ID
to every command that issues some procedure over GATT.
So connections command will list every active connection with connection
ID and remote address.
Update connect/disconnect commands for multiple connections

Some changes on connect and disconnect commands to support multiple
connections.
Update search-svc command for multiple connections

The connection ID was added in search-svc command to indicate which
device will be prompt for primary/secondary services.

clear_list_cache() also was updated to receive the connection ID and
clean only the respective services list.
Update included command for multiple connections

The connection ID was added in included command to indicate which device
will be prompt for included services.
Add support for multiple connections in find_svc()

The connection ID is need to search for a service from a specific
device.
Update characteristics command for multiple connections

The connection ID was added in characteristics command to indicate which
device will be prompt.
Update char-desc command for multiple connections

The connection ID was added in char-desc command to add support for
multiple connections.
Update read-char command for multiple connections

The connection ID was added in read-char command to add support for
multiple connections.
Update read-desc command for multiple connections

The connection ID was added in read-desc command to add support for
multiple connections.
Update rssi command for multiple connections

The connection ID was added in rssi command to add support for multiple
connections.
Update write-req-char/write-cmd-char commands for multiple connections

The connection ID was added in write-req-char/write-cmd-char commands to
add support for multiple connections.
Update write-desc command for multiple connections

The connection ID was added in write-desc command to add support for
multiple connections.
Update reg and unreg notification commands for multiple connections

The connection ID was added in reg and unreg notification commands to
add support for multiple connections.
Remove old connection variables

Now we have an array to handle multiple connections, so we don't need
these variables anymore.
Bump version, update ChangeLog and README