remove flat_index fully

[oxarbitrage]

i was trying to remove the last flat_index we have in our code. this is an index around block_summary_object. this is for issue #325
i had problem on making this change so i asked the help of @pmconrad . all the credit goes to him, thank you peter!

i compiled the patch and started the node. replayed the chain and the new index started to populate fine.

api calls to block_summary_objects show correct results:

> {"jsonrpc": "2.0", "method": "get_objects", "params": [["2.8.1"]], "id": 1}
  
< {"id":1,"result":[{"id":"2.8.1","block_id":"00250001f2f73c323ca7f825eb64174c943d4ec5"}]}
> {"jsonrpc": "2.0", "method": "get_objects", "params": [["2.8.100"]], "id": 1}
  
< {"id":1,"result":[{"id":"2.8.100","block_id":"00260064f382907efd4920d94116ecfed35dca39"}]}
> {"jsonrpc": "2.0", "method": "get_objects", "params": [["2.8.10000"]], "id": 1}
  
< {"id":1,"result":[{"id":"2.8.10000","block_id":"00262710e58766622aebc60b3e1f792a60f311a1"}]}
> 

now the chain is fully in sync, all good from what i can see here in an initial test. more testing/comments/etc are appreciated.

[pmconrad]

ACK, but not sure if that counts. :-)