Fix Blockchain::get_tail_id() to set parameter to last block number instead of height
Fix Blockchain::get_tail_id() to set parameter to last block number instead of height

This reflects the behavior of blockchain_storage::get_tail_id().

Fixes #27 so that RPC method getlastblockheader works.

ranok/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero