block storage GetLastCommittedBlockHeight api is problematic

[jlevison]

it returns the block height and the timestamp in a struct
using it requires to box and unbox the result each time - makes little sense when all we need is the block height

in actual, it is not used today in the system
what is used is LastBlockHeight(), 94 times i believe

can we change it?

[OdedWx]

Depends on: #109

We added the block time for CallContarcts that were called by the PublicAPI.
Currently PublicAPI doesn't use the block height/time as the flow was changed to get the height/time from the state storage. Let's close #109 first.