Change variable name.

  lastEventId ->
dbLastEventId

Because, I'm preparing to check last "eventid" in Zabbix Server.
So, conventional name cause an accident.
Fix wrong function type.

 int64_t ->
uint64_t
Fix wrong variable type.

 int64_t ->
uint64_t
Change error indication.
Add string type variable "strLastEventId" to store json data.

I think first I can store json data to uint64_t variable.
But, I understood I could not do it.
Add appoint point to start reading json.
Add change from string type to uint64_t type.
Add store last "eventid" in Zabbix Server.