Add isAsync() for All Handlers

[vic-ma]

Fixes #312

Handler	Async?	Notes
AuthPostHandler	No
ClientUrlGetHandler	No	A unit test expects it to be async.
GrafanaDashboardUrlGetHandler	No
GrafanaDatasourceUrlGetHandler	No
RecordingDeleteHandler	No
RecordingGetHandler	Yes	ctx.vertx().filesystem().exists() is async.
RecordingsGetHandler	No
RecordingsPostHandler	Yes	validateRecording() and saveRecording() are async, but this handler already has isAsync() set to false.
RecordingUploadPostHandler	No	Already has isAsync() set to false.
ReportGetHandler	No	Already has isAsync() set to false.
TargetEventsGetHandler	No
TargetRecordingDeleteHandler	No
TargetRecordingGetHandler	No	Already has isAsync() set to false.
TargetRecordingOptionsGetHandler	No	Already has isAsync() set to false.
TargetRecordingOptionsPatchHandler	No
TargetRecordingPatchHandler	No	Already has isAsync() set to false.
TargetRecordingsGetHandler	No
TargetRecordingsPostHandler	No	Already has isAsync() set to false.
TargetRecordingUploadPostHandler	No	Already has isAsync() set to false.
TargetReportGetHandler	No	Already has isAsync() set to false.
TargetsGetHandler	No
TargetSnapshotPostHandler	No
TargetTemplateGetHandler	No
TargetTemplatesGetHandler	No
TemplateDeleteHandler	No
TemplatesPostHandler	No
CertificatePostHandler (V2)	No	Already has isAsync() set to false.
TargetEventsSearchGetHandler (V2)	No	Already has isAsync() set to false.
TargetRecordingOptionsListGetHandler (V2)	No	Already has isAsync() set to false.
TargetSnapshotPostHandler (V2)	No

AuthPostHandler, GrafanaDashboardUrlGetHandler, and GrafanaDatasourceUrlGetHandler are not async, but they're all simple handlers, so I've set their isAsync() values to true anyway, as suggested.

ClientUrlGetHandler doesn't seem to be async from what I can tell, but there's a unit test that expects it to be async, so I didn't change it.

RecordingsPostHandler seems to have async helper functions, but it already had its isAsync() set to false, so I didn't change it.

Edit: TargetRecordingsPostHandler didn't actually have isAsync() set to false already, but it does appear to be synchronous.

[andrewazores]

Nice work. I read over all of your notes and glanced over the implementations and I agree with all of your findings. Please also prepare a PR to backport this to the v1 branch.