Update ReentrancyGuard for Istanbul Hard Fork

[ericDeCourcy]

Changes:
Added L37, _guardCounter = 1;

Rationale:
The planned Istanbul Hard Fork will implement EIP 2200, which implements "net gas metering" for sstore operations. If the final value of _guardCounter is unchanged relative to the original value of it, a gas refund will be applied and charges for changing the value of _guardCounter will effectively not exist. This ends up being cheaper than the current implementation ONLY AFTER Istanbul. Before Istanbul, the added line actually ends up costing more gas.

Note that if _guardCounter is 0 initially, the initial cost and subsequent refund will both be larger than if _guardCounter is 1 initially. Although in both cases, the net gas cost (gasCost - gasRefund) are equal, it's better in terms of cost to have both the gas cost and refund smaller, as there is some limit to the percentage of a gas refund that can actually be realized.

[nventuro]

Thanks a lot @ericDeCourcy! Could you add an entry for this change on the changelog file?

[nventuro]

Thanks! I took the liberty of changing it up a bit so that the improvements introduced by this PR are more evident by the reader, who may not be aware of the implications of the actual code change.

I also added a note on the documentation for the module, to entice people to try out the new version.