support multiple log gatherers using the new multi-gatherer feature of foolscap

[zooko]

Once Foolscap ticket #176 is fixed to accept multiple log gatherers, we could use that functionality to let Tahoe-LAFS users configure multiple log gatherers in their tahoe.cfg file.

There are some questions about dependency versioning: do we wait until the new version of foolscap is widespread and then raise the version requirement that Tahoe-LAFS has on foolscap to that version? Or do we detect whether the version of foolscap is present can handle multiple log gatherers and emit an error message with instructions to upgrade if it is not new enough and the Tahoe-LAFS user has configured multiple log gatherers?

This will fix a regression in Tahoe-LAFS (#1385).