Add offline mode

[haadcode]

This PR adds an options flag to start an OrbitDB instance in offline mode.

Inside OrbitDB, if options.offline: true is set, Pubsub won't be started, thus no exchange of updates will happen.

This is a useful feature for example when starting IPFS nodes offline (with new IPFS({start: false})). For example, we should optimize some of the tests to use offline IPFS and OrbitDB nodes for faster startup times and to decrease overall testing time.

The flag is documented as:

• offline (boolean): Start the OrbitDB instance in offline mode. Databases are not be replicated when the instance is started in offline mode. If the OrbitDB instance was started offline mode and you want to start replicating databases, the OrbitDB instance needs to be re-created. Default: false.

And would be used like:

const orbitdb = await OrbitDB.createInstance(ipfs, {offline: true})
...

Note, as per the API doc, if the OrbitDB instance was started offline mode and you want to start replicating databases, the OrbitDB instance needs to be re-created. There's currently no support for "going online" if the node has been started in offline mode, but this is something we can add in the future.