flood of recovery requests

[santicomp2014]

This issue was originally raised in #2205.

After #2202 was merged, flooding of recovery attempts is still occurring under a different set of conditions.

The current implementation sets the publisher in the context when downloading content; so that content recovery process is triggered only when the publisher was explicitly set in the download query/request. But it seems this is not enough to fix all of the flooding.

When a made-up hash is requested and the publisher is set, the flooding also occurs.

Something similar occurs when setting a made-up publisher.

Verify that the test scenarios behave as specified, without flooding in any case:

Chunk	Publisher	Expected Result
Valid Chunk	Valid Publisher	Recovery Successful
Invalid Chunk	Valid Publisher	Recovery Unsuccessful
Valid Chunk	Invalid Publisher	Recovery Unsuccessful
Invalid Chunk	Invalid Publisher	Recovery Unsuccessful

(valid/invalid means existing/non-existing)