storage/pin: fix getAllChunksFromDB test function

[janos]

This PR fixes collectPinnedChunks method in test API which makes tests in storage/pin flaky:

--- FAIL: TestPinCollectionUploadEncrypted (0.17s)
    pin_test.go:84: Expected number of chunks to be 38, but is 40
FAIL
FAIL    github.com/ethersphere/swarm/storage/pin    1.031s

There are two issues.

One is the asynchronous getting of chunks which wait for only 10ms (as context timeout) for all chunks to be retrieved from the database, which may be problematic on Travis when it slows down. Even the context done error was not handled in the goroutine, only a return, resulting incomplete chunks slice returned from collectPinnedChunks. This PR uses LastPullSubscriptionBinID to correctly get the right number of chunks and removes goroutines as they only introduce complexity for such small number of chunks. Test are not slowed down.

The second issue is that the for loop had an invalid break condition on chunk.MaxPO, not including the max proximity order (last value).

[mortelli]

LGTM.

the timeout and end condition for the loop changes seem OK. besides that, the old implementation seems a bit too complex, since it uses both channels as well as mutexes. it seems it didn't handle errors either.

due to my unfamiliarity with the code, i would prefer to leave the approval to someone who is more knowledgeable in the storage and pin packages (maybe @jmozah or someone from core).