Fix re-entrance

[syntrust]

Issue to address:

• Avoid reentrancy attack when giving reward to miner #107

Solution:

• use transient storage to save gas (100 gas for TSTORE/TLOAD)
• currently https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/ReentrancyGuardTransient.sol has not been released, so copy the source code for the time being and wait for the next release to remove it.

Tests:

• First mocked a re-entrance attack to drain funds from the storage contract.
• Then verify that the noReentrant modifier will revert the same test.