Converted plugin executor to ScheduledExecutorService.

api/net/sf/briar/api/plugins/BatchPluginFactory.java

api/net/sf/briar/api/plugins/StreamPluginFactory.java

components/net/sf/briar/plugins/PluginManagerImpl.java

components/net/sf/briar/plugins/PluginsModule.java

...nents/net/sf/briar/plugins/bluetooth/BluetoothPlugin.java

...et/sf/briar/plugins/bluetooth/BluetoothPluginFactory.java

...et/sf/briar/plugins/file/RemovableDrivePluginFactory.java

components/net/sf/briar/plugins/socket/LanSocketPlugin.java

...nents/net/sf/briar/plugins/socket/SimpleSocketPlugin.java

...et/sf/briar/plugins/socket/SimpleSocketPluginFactory.java

components/net/sf/briar/plugins/socket/SocketPlugin.java

test/net/sf/briar/plugins/PluginManagerImplTest.java

test/net/sf/briar/plugins/bluetooth/BluetoothClientTest.java

test/net/sf/briar/plugins/bluetooth/BluetoothServerTest.java

test/net/sf/briar/plugins/socket/LanSocketClientTest.java

test/net/sf/briar/plugins/socket/LanSocketServerTest.java

test/net/sf/briar/plugins/socket/SimpleSocketPluginTest.java