Move thread activation to start method

[samamorgan]

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This moves activation of LiveServer.thread to a method. My primary motivation here is to delay thread activation so I can change the parameters of the thread in an overriden live_server fixture. This provides a work-around to issue #294 using the following override:

import pytest
from pytest_django import live_server_helper
from pytest_django.lazy_django import skip_if_no_django


@pytest.fixture(scope="session")
def live_server(request):
    skip_if_no_django()

    addr = (
        request.config.getvalue("liveserver")
        or os.getenv("DJANGO_LIVE_TEST_SERVER_ADDRESS")
        or "localhost"
    )

    server = live_server_helper.LiveServer(addr)
    server.thread.static_handler = _StaticFilesHandler
    server.start()
    request.addfinalizer(server.stop)

    return server