Moved some code out of swift.obj.replicator

This will be needed in future replication work to avoid circular
imports.

I used swift.obj.base as the module name just because we seemed to
avoid putting code in __init__.py files so far and I didn't want to
buck the trend.

I would love to see other obj things like *_metadata and DiskFile
move into swift.obj.base as well and swift.obj.server just be the
WSGI server logic, but I'll leave that for the future.

I have changed the tests as little as possible (just the references
to where they get the code to test) to show the refactor has not
broken anything. I did add a test for tpool_reraise since there was
none before.

There will be a follow on patch for moving the tests to their new
location(s). I figured I'd wait to put the bikes in the shed until
everyone's done painting it.

Change-Id: I32b4ac88be21eb76c877d3f4cc1e6ac33304835b
Merge "Moved some code out of swift.obj.replicator"