move controlport/logport to a separate Tub (and separate port)

[warner]

(splitting this out of #2491)

Currently, the "controlport" and logport both use the same Tub as everything else (which, since we landed the #2759 one-Tub-per-server change, means "the same Tub as the storage server"). We want to not open a listening port unless we're running a storage server, and we're requiring the operator to tell us what the externally-visible location of that server is (rather than attempting to determine an IP address automatically), so the controlport/logport should be moved to a separate Tub.

This new control/log Tub should allocate a port (unless one is explicitly configured in tahoe.cfg) and should advertise a location of 127.0.0.1 (unless overridden in tahoe.cfg). That will enable both the automated tests like check_memory.py, and the common flogtool tail NODEDIR/private/logport.furl idiom.

If folks want to debug their nodes from a distant machine, they'll need to set tahoe.cfg to provide a location for their logport, so logport.furl will get a real hostname/ipaddr, instead of 127.0.0.1

[dawuud]

ok here's my attempt to solve this one: ​https://github.com/david415/tahoe-lafs/tree/2794.separate_tub_control_log_port.0

i think it might work but the coverage test fails because i didn't write a test for the new code?

[Brian Warner <warner@…>]

In 7d692d0/trunk:

Split control and log to two separate tubs, remove control knobs.

This also removes the tahoe.cfg keys that would have configured the
control-port. And it deletes the logport.furl file before asking the Tub
to re-create it, because we're now using an ephemeral Tub (so we're not
persisting the private key, so the tubid will change each time).

closes ticket:2794