Added IpcIo DiskIO module for communication with remote disk processes via UDS.

Added IpcIo DiskIO module for communication with remote disk processes via UDS.
Used IpcIo for Rock Store filesystem module.

Added StrandSearch API: Workers use it to ask Coordinator for the right
address (i.e., kid identifier) of the disk process for a given cache_dir path.
If Coordinator does not know the answer, it waits for more disk processes to
register. Implemented using generic tagging of kids (StrandCoord) and
searching for the right tag.

Raised UDS message size maximum to 36K in order to accommodate non-trivial
rock store I/O while we are using UDS messages for I/O content.

Fixed shutdown handling broken by hiding cache_dirs from Coordinator while
switching IamPrimaryProcess() logic to use NumberOfKids() which needs
cache_dir count.