Improve case consistency for initialisms in the tunnel server package
As it is suggested in #468 (closed) to handle the issue in multiple MRs, this MR does not include all of the needed renamings, but only the following:
- Rename findRequestsForAgentId to findRequestsForAgentID
- Rename findRequestsByAgentId to findRequestsByAgentID
- Rename tunnelsByAgentIdHashKey to tunnelsByAgentIDHashKey
- Rename tunnelsByAgentId to tunnelsByAgentID
Apart from !1204 (merged), this MR does not seem to have any other blockers
See https://go.dev/wiki/CodeReviewComments#initialisms for more information.