Avoid addtional timestamp conversion to set state
Since we already have the timestamp, we can pass it on to the State
object and avoid the additional timestamp conversion which can be as
much as 30% of the state write runtime.
Since datetime objects are limited to microsecond precision, we need
to adjust some tests to account for the additional precision that we
will now be able to get in the database
Since someone websocket subscriptions will use an iden of 2 for
state_changed event (most comment), and some will use another
number for all events, the cache would not be used because the
iden number was different. We now cache only the event and
use a fast replace to insert the iden number into the serailized
response.
Since most of the json serialize work for the websocket was done
multiple times for the same message, we can avoid the overhead
of serializing the same message many times (once per websocket
client) with a cache.
