dht: make main store a vector<unique_ptr<Storage>>

Since the elements inside are going to be moved constantly during the life of
the node, this design is better. Also, it works around a crash when the store is
resized (elements inside moved) at some point (always happenned the 3rd time)
for some reason. This bug was known when compiled with LLVM >7.3.0 (>clang-703.0.31).