Rework and correct getheaders handling.

This commit reworks the getheaders handling a bit to clean it up and match
the reference implementation handling.  In particular, in addition to the
normal handling where headers starting after the block locator up to the
stop hash are served, when no locator hashes are provided, the stop hash
acts as a way to specifically request that header.  Next, an empty headers
message is sent when no hashes provided by the block locator can be
found.  Finally, there was a bug that was limiting the number of headers
that could requested at once to 500 instead of the expected 2000.