exchange: introduce functionality to lock repos during each run
This introduces a exchange lock which writes a `.hg/scmperf-lock` file. This will help us in someway which I am still not sure.
check the repos directory afterwards
ERROR: test-smoke-test.t output changed
Failed test-smoke-test-python3.t: output changed
Failed test-smoke-test.t: output changed
# Ran 11 tests, 0 skipped, 2 failed.
python hash seed: 264865318section_end:1582035349:build_scriptsection_start:1582035349:after_scriptsection_end:1582035359:after_scriptERROR: Job failed: exit code 1