PC tree / tensor DM round trip is far too slow.

[brettviren]

The current implementation of the PC tree Tensor Data Model I/O is very slow. A 2M tensor PC tree can be expected. A 1M node tree made from 1M "janky" test tracks takes 7 seconds to generate, 23 seconds to convert to tensordm, 14 seconds to index the tensors and 28 seconds to convert back to pctree with the index.

From google-perftools, about half of the time is spent constructing and destructing JsonCPP::Value.

Even if that magically goes away, it would be still too slow.

I believe the core problem is that the pctree TDM results in a large number of small files/objects and the per file/object I/O overhead is killing performance. The implementation must either remove this overhead or the pc tree tensor data model must be redefined to allow fewer, more monolithic objects.