Don't discard unwanted blocks we actually want

[Stebalien]

Currently, we discard blocks if they're not in the wantlist. However, we should probably accept them anyways if they're in the interest set.

[dirkmc]

Adding a little more context for my own understanding:

The session has an InterestedIn() function that returns true for any want that either

1. Is pending being requested (until there is a free space in the live request list)
2. Has been requested (in the live request list) but no response has yet been received

The WantManager has an IsWanted() function that returns true for any want that has been requested but no response has yet been received (same as 2 above).

If we happen to receive this block while it is still in the pending state (same as 1 above) we will not add it to the blockstore