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, PortalTravelAgent now does a more thorough check of blocks which could intersect with the player, and adjusts the exit location to be within the portal rather than on the edge more than vanilla does, such that the player should never intersect with blocks immediately after using a portal, and hence will never be taken to above the target portal.