Adding Neo.Runtime.GetTime and Neo.Runtime.GetCurrentBlock

[localhuman]

for issue #114

• adds Neo.Runtime.GetTime
• adds Neo.Runtime.GetCurrentBlock
• adds property CurrentBlock to Blockchain. This retrieves the currently persisting block if a block is currently being persisted, otherwise it returns the highest block

If either of these API are called during TriggerType.Verification, the most recent blockchain on the block is returned. If it is called during TriggerType.Application, it will be the currently persisting block.

[shargon]

Maybe in the else was good option set persisting_block=GetBlock (Height)

[localhuman]

I want persisting_block to be null unless execution is in the middle of LeveldbBlockchain.Persist

[erikzhang]

I'm not going to accept Neo.Runtime.GetCurrentBlock because we're going to implement a pre-execution mechanism for applications.
In a pre-execution, the hash field and next_consensus field of the persisting_block are not determined. If we want to ensure that the result of the pre-execution is consistent with the result of normal execution, we can't provide an API for getting persisting_block.

[localhuman]

Will remove GetCurrentBlock.
for GetTime, the issue says to get the current block and add 15s. Do we want to add that, given that the time may not be exact?

[erikzhang]

For GetTime:

1. If it's running under Application trigger, return the timestamp of the persisting_block.
2. If it's running under Verification trigger, return the timestamp of latest block and add Blockchain.SecondsPerBlock.