Add pending block operations

Problem: a lot of problems are created by tracking the importing progress on multiple tables.
Querying and updating involves as such heavy operations and this is also the cause for inconsistencies.

Solution: use a new table to track the progress of block's data indexing.
By using a specific table, whose rows are deleted when there are no more pending operations on the relative block, we make joins quick and keep them to a minimum.
This is also meant to simplify all inserts and updates and only perform deletes when strictly necessary.

apps/explorer/lib/explorer/chain/pending_block_operation.ex

@@ -16,7 +16,7 @@ defmodule Explorer.Chain.PendingBlockOperation do
   @type t :: %__MODULE__{
           block_hash: Hash.Full.t(),
           fetch_internal_transactions: boolean()
-        }
+  }
 
   @primary_key false
   schema "pending_block_operations" do