Improve coordinator time

[AdityaSripal]

Currently the blocktime does not advance at all on NextBlock. It is only incremented by the coordinator so that it can be incremented on all testchains at the same time.

Thus, the testing suite keeps track of a global time. This is to prevent errors on UpdateClient when the update header time is ahead of the block time. However, the use of a global clock is inelegant and does not simulate the increasing time of real blockchains.

It would be ideal for chains to increment their time each block by a set amount, and then ensure the header from future bug does not occur some other way.

[colin-axner]

With the update to ABCI++ #3707, we could replace the managed coordinator time with usage of time.Now(). We would need to still devise a way for chains to magically fast forward the clock time if they wanted to test scenarios in the future (like for gov proposals)

In the case where we use time.Now(), before committing and executing the current block, we can update the header time to be the current time. I think this would to occur regardless for globally managed time

Alternatively, instead of time.Now and a coordinator, we could simply keep the global time and allow chains to draw from it without going through a coordinator (maybe have a pointer to a time object which is given to each chain). We would likely still need to update the header before block commitment/execution

One thing I haven't determined yet is if there will be side affects between block proposal and execution where the context may reference a time which will be changed upon execution

[colin-axner]

I think maybe the improvement here would be to turn the current time object in the coordinator into a pointer to which chains receive access. Upon executing a block, they can then increment the global time. As stated above, they will likely need to refresh the time in their header before committing/executing a block