Accessing non-existing chunk floods network with requests

[Fraser999]

Migrated from maidsafe-archive/MaidSafe#57

When we try to read a file which has a chunk missing, for each increment of the offset pointer that falls within the chunk we re-try to retrieve it, only to fail every time.

Encrypt needs to be aware which chunks it already tried to retrieve unsuccessfully to prevent this. Since we don't know whether the chunk is only unavailable temporarily, we need to re-try after a certain period of time.