Add a dir-entry setting to volumes.config

[zwoop]

The idea / purpose of this is to allow for creating a volume which has a different configuration for the number of directory entries. The use case would be in a mixed environment where you have a mix of very large and very small objects, and don't want to waste a lot of memory for unused directory entries.

Example use:

volume=1 scheme=http size=20%
volume=2 scheme=http size=20%
volume=3 scheme=http size=20%
volume=4 scheme=http size=20%
volume=5 scheme=http size=20% avg_obj_size=1024

And then assign volume5 to one or many hosts / domains via hosting.config.

Before using avg_obj_size, we'd get the same number of directories per volume:

root@frigg:/opt/ats-10# ./bin/traffic_ctl  metric match 'direntries.total'
proxy.process.cache.direntries.total 4183592
proxy.process.cache.direntries.used 0
proxy.process.cache.volume_1.direntries.total 835072
proxy.process.cache.volume_2.direntries.total 835072
proxy.process.cache.volume_3.direntries.total 835072
proxy.process.cache.volume_4.direntries.total 839188
proxy.process.cache.volume_5.direntries.total 839188

after enabling the new configuration on volume5:

root@frigg:/opt/ats-10# ./bin/traffic_ctl  metric match 'direntries.total'
proxy.process.cache.direntries.total 29700544
proxy.process.cache.volume_1.direntries.total 835072
proxy.process.cache.volume_2.direntries.total 835072
proxy.process.cache.volume_3.direntries.total 835072
proxy.process.cache.volume_4.direntries.total 839188
proxy.process.cache.volume_5.direntries.total 26356140