Make FailingSnapshotManager Thread Safe

[gabber235]

Problem Description

When running multiple path searches at the same time from different threads, the FailingSnapshotManager throws the following error:

java.util.ConcurrentModificationException: null
	at java.util.HashMap.computeIfAbsent(HashMap.java:1221) ~[?:?]
	at org.patheloper.model.snapshot.FailingSnapshotManager.processChunkSnapshot(FailingSnapshotManager.java:108) ~[Typewriter.jar:?]
	at org.patheloper.model.snapshot.FailingSnapshotManager.getChunkSnapshot(FailingSnapshotManager.java:96) ~[Typewriter.jar:?]
	at org.patheloper.model.snapshot.FailingSnapshotManager.fetchBlock(FailingSnapshotManager.java:59) ~[Typewriter.jar:?]
	at org.patheloper.model.snapshot.FailingSnapshotManager.getBlock(FailingSnapshotManager.java:116) ~[Typewriter.jar:?]
	at org.patheloper.model.snapshot.FailingSnapshotManager$RequestingSnapshotManager.getBlock(FailingSnapshotManager.java:168) ~[Typewriter.jar:?]
	at org.patheloper.model.pathing.pathfinder.AbstractPathfinder.isBlockUnreachable(AbstractPathfinder.java:112) ~[Typewriter.jar:?]
	at org.patheloper.model.pathing.pathfinder.AbstractPathfinder.initialChecksFailed(AbstractPathfinder.java:104) ~[Typewriter.jar:?]
	at org.patheloper.model.pathing.pathfinder.AbstractPathfinder.findPath(AbstractPathfinder.java:71) ~[Typewriter.jar:?]
	at org.patheloper.model.pathing.pathfinder.AStarPathfinder.findPath(AStarPathfinder.java:34) ~[Typewriter.jar:?]

Proposed solution is to change the data structure of the SNAPSHOTS_MAP from a unsafe Map to a thread safe ConcurrentMap.

This pull request does just that.

[olijeffers0n]

Hi, thanks for this! Could you target our trunk branch with this PR please

[olijeffers0n]

LGTM - @GodCipher can final approval and merge

[GodCipher]

Hi, thanks for the fix tho! Once the import is removed we will merge this into pathetic :)