Update EmailLog table only when Email has been physically sent

Currently record in EmailLog table is created after
"kEmailSendingHelper::Deliver" method is called. This approach is correct in
most cases, but when email is placed into email queue and is sent later via
cron, then log record is created immediately.

I propose to create "kEmailSendingHelper::setLogData" method, that should
accept EmailLog table fields (e.g. fromuser, event, EventParams) and store
all given data in "kEmailSendingHelper::_logData" attribute. Add new field
LogData to EmailQueue table and store "kEmailSendingHelper::_logData"
attribute contents in case, when email is placed into queue. In case, when
Deliver method is called it will use log data stored before to create log
record. Method "kEmailSendingHelper::Clear" should also clear log data
stored before.