Prevent stuck block download in large reorganisations

In cases of very large reorganisations (hundreds of blocks), a situation
may appear where an 'inv' is sent as response to a 'getblocks', but the
last block mentioned in the inv is already known to the receiver node.
However, the supplying node uses a request for this last block as a
trigger to send the rest of the inv blocks. If it never comes, the block
chain download is stuck.

This commit makes the receiver node always request the last inv'ed block,
even if it is already known, to prevent this problem.
Prevent stuck download: correct solution

Pull request #948 introduced a fix for nodes stuck on a long side branch
of the main chain. The fix was non-functional however, as the additional
getdata request was created in a first step of processing, but dropped
in a second step as it was considered redundant. This commits fixes it
by sending the request directly.
Hopefully final fix for the stuck blockchain issue

Immediately issue a "getblocks", instead of a "getdata" (which will
trigger the relevant "inv" to be sent anyway), and only do so when
the previous set of invs led us into a known and attached part of
the block tree.
Merge pull request #24 from khalahan/orphan-blocks-stuck-chain

Orphan blocks stuck chain