The amount of gas available for tx execution was tracked in the
StateObject representing the coinbase account. This commit makes the gas
counter a separate type in package core, which avoids unintended
consequences of intertwining the counter with state logic.
* Update => SyncIntermediate
* Added SyncObjects
SyncIntermediate only updates whatever has changed, but, as a side
effect, requires much more disk space.
SyncObjects will only sync whatever is required for a block and will not
save intermediate state to disk. As drawback this requires more time
when more txs come in.
