Thread pools

seems to do the trick, I can see stuff like:

```
              18322 /bin/sh /var/tmp/opengrok-0.12/bin/OpenGrok index /ws-local-0.12
                18345 /usr/java/bin/amd64/java -XX:-UseGCOverheadLimit -Xmx8192m -server -Dorg.openso
                  24582 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  24586 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  24606 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  24697 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  24771 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  24776 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  25064 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  25288 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  25508 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  26292 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  26726 /usr/bin/python2.6 /usr/bin/hg log --template changeset: {rev}:{node|short}\nus
                  29776 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29777 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29779 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29780 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29782 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29784 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29785 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29786 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29788 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
                  29789 /usr/bin/python2.6 /usr/bin/hg log -f --template changeset: {rev}:{node|short}\
```

The `hg log` commands with `-f` are obviously for the renamed files so both top-level repo histories and renamed files are chugging along nicely which was not the case in the previous run without the fix.