Add functionality to load objects from ZODB as saved by py wendelin.core.
Mostly straightforward code.
The main part is in zblk.go .

Contrary to python implementation, go can load ZBlk1's subobjects in
parallel, which, given scalable ZODB storage, can be significantly
faster compared to serially loading all ZData subobjects as py code
does.

TODO test wrt data saved by Python3.

Some preliminary history:

878b2787 X draft loading
bf9a7405 X No longer rely on ZODB cache invariant for invalidations
0d62b05e X Adjust to btree.VGet & friends signature change to include keycov in visit callback
b74dda09 X Start switching Track from Track(key) to Track(keycov)