* cs: check for SkipTimeoutCommit or wait timeout in handleTxsAvailable
Previously, if create_empty_blocks was set to false, TM sometimes could
create 2 consecutive blocks without actually waiting for timeoutCommit
(1 sec. by default).
```
I[2019-08-29|07:32:43.874] Executed block module=state height=47 validTxs=10 invalidTxs=0
I[2019-08-29|07:32:43.876] Committed state module=state height=47 txs=10 appHash=F88C010000000000
I[2019-08-29|07:32:44.885] Executed block module=state height=48 validTxs=2 invalidTxs=0
I[2019-08-29|07:32:44.887] Committed state module=state height=48 txs=2 appHash=FC8C010000000000
I[2019-08-29|07:32:44.908] Executed block module=state height=49 validTxs=8 invalidTxs=0
I[2019-08-29|07:32:44.909] Committed state module=state height=49 txs=8 appHash=8C8D010000000000
I[2019-08-29|07:32:45.886] Executed block module=state height=50 validTxs=2 invalidTxs=0
I[2019-08-29|07:32:45.895] Committed state module=state height=50 txs=2 appHash=908D010000000000
I[2019-08-29|07:32:45.908] Executed block module=state height=51 validTxs=8 invalidTxs=0
I[2019-08-29|07:32:45.909] Committed state module=state height=51 txs=8 appHash=A08D010000000000
```
This commit fixes that by adding a check to handleTxsAvailable.
Fixes#3908
* update changelog
* schedule timeoutCommit if StartTime is in the future
or SkipTimeoutCommit=true && we DON'T have all the votes
* fix TestReactorCreatesBlockWhenEmptyBlocksFalse
by checking if we have LastCommit or not
* address Ethan's comments