This refactors the code that locates blocks (inventory discovery) out of
server and into blockchain where it can make use of the new much more
efficient chain view and more easily be tested. As an aside, it really
belongs in blockchain anyways since it's purely dealing with the block
index and best chain.
Since the majority of the network has moved to header-based semantics,
this also provides an additional optimization to allow headers to be
located directly versus needing to first discover the hashes and then
fetch the headers.
The new functions are named LocateBlocks and LocateHeaders. The former
returns a slice of located hashes and the latter returns a slice of
located headers.
Finally, it also updates the RPC server getheaders call and related
plumbing to use the new LocateHeaders function.
A comprehensive suite of tests is provided to ensure both functions
behave correctly for both correct and incorrect block locators.
