Added BoundedExecutor and documented executor policies.

...et/sf/briar/api/transport/IncomingConnectionExecutor.java

components/net/sf/briar/db/DatabaseModule.java

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

components/net/sf/briar/protocol/ProtocolModule.java

...nents/net/sf/briar/protocol/VerificationExecutorImpl.java

...ents/net/sf/briar/transport/ConnectionDispatcherImpl.java

...ents/net/sf/briar/transport/ConnectionRecogniserImpl.java

components/net/sf/briar/transport/TransportModule.java

util/net/sf/briar/util/BoundedExecutor.java