big update to configuration and flow logic
big update to configuration and flow logic

Several joint changes:
1. Configuration: remove default interval params and default database.
There are no longer defaults for these in config, only in code.

2. In [queue] and [history] sections, allow any number of source = spec
lines. This is an expansion: we can define multiple sources for both
schedd queues and history files in the config file now, in the same way
as we used to define multiple history files (but only on the command
line).

3. Allow complete supercession of config file source/spec pairs on
command line.

4. Converge handling of queue and history code.  Almost ready to
make these the same code path.

5. Normalize and simplify subprocess/thread management, making it easier
to cleanly do several tasks in parallel.  Each source for queue and
history will get its own thread of execution.

This overall gives us a significant expansion of capability while
simplifying the configuration and application logic rather a lot.  (It
might look more arcane, but it's still logically simpler.)

Example configurations:
[queue]
local = foobar

local = foo, bar

local = foo, bar, interval=180

condor.example.com = foo, bar

DHTC-Tools/crow,DHTC-Tools/crow,DHTC-Tools/crow,DHTC-Tools/crow