ipfs: retrieving data does not work with large(r) blocks

[liamsi]

While testing with the DAS light client (#323) we discovered the following behavior:

Sampling, or more general, reading data from another node that had previously called PutBlock only succeeds for small blocks (smaller than 16 kb?).

I don't know the reason yet. @Wondertan started working on a unit test for this that confirms this behavior even when using a mock network and two ipfs nodes.

This needs to be fixed independently from the light client.

[liamsi]

The issue was not this (celestiaorg/nmt#37 happened only during tests) but #336 (fixed by @evan-forbes). Sampling works just fine now but we should still add tests using mock ipfs nodes and e2e tests as well.

For one such mock test see: 1522021#diff-39653ba45e0af8a1a11577bda9cf5b939d78758cebe07da377470aaaba2e0b1aR307

[liamsi]

closing in favor of #349 which is only about improving the tests