Use the result of EntityPortalExitEvent. Fixes BUKKIT-3559.
Always use the set exit location for portal events. Fixes BUKKIT-3555.
Fix players spawning above portals. Fixes BUKKIT-3542.

Vanilla does not check for blocks in which the player could
suffocate when changing dimension, so portals will happily spawn
players in blocks when using a portal under certain
circumstances. However, we currently check for these instances
and move the player up until they will not suffocate. This means
that players can sometimes be taken to above the target portal,
making it seem as if a portal was not created. Instead, we now
disable this suffocation check when moveToWorld is called from
changeDimension, mirroring vanilla behavior more accurately.
Always return a TravelAgent; Fixes BUKKIT-3541

Recent changes caused PlayerPortalEvent to suddenly return null
unexpectedly and could end up in NPEs resulting that did not before.
This commit addresses that situation by always ensuring a TravelAgent
instance is returned.

The TravelAgent for world 0 is returned arbitrarily in an effort to
compensate for plugins that are implementation dependent and expect some
form of a TravelAgent to be accessible in the event at all times.