condense parallel requests with the same hashes to one

condense parallel requests with the same hashes to one

It shouldn't be too common, but sometimes people have multiple mirrors
in the sources or otherwise repositories with the same content. Now that
we gracefully can handle multiple requests to the same URI, we can also
fold multiple requests with the same expected hashes into one. Note that
this isn't trying to find oppertunities for merging, but just merges if
it happens to encounter the oppertunity for it.

This is most obvious in the new testcase actually as it needs to delay
the action to give the acquire system enough time to figure out that
they can be merged.