This adds a function to the sancov_merger that merges the
output files of one swarming shard in parallel. This will
be called from the infrastructure when collecting/merging
swarming results.

The tool will be called subsequently for each shard. On
the first call, the target dir will be empty and the sancov
files will just be moved.

BUG=chromium:568949
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1782843002

Cr-Commit-Position: refs/heads/master@{#34678}