istanbul/core: fixed potential memory leak in backlogs

[trung]

backlogs is the map with keys which are pointers to Validator. In a scenario when node re-initializes the validator set, new set of pointers are instantiated causing new entry being added into backlogs when subsequent storeBacklogs() is called.

[fixanoid]

Fixes #481

[marcosio]

Hi guys, we are introducing this fix in our nodes and:

1. With 3 nodes, 1 validator: The fix works fine, the validator node has the minimum amount of memory, and if we stress the network with a lot of transactions, it is resilient.

2. With 48 nodes, 7 validators: We set 2 nodes with the modifications. Both of them increase her amount of memory for another reason.

Any idea of another issue that can resolve this?

What can we test for identifying the origin of this memory leak?

Meanwhile, we will change our test configuration in the test environment with 2 validators and 2 regular nodes.