provider/aws: Retry finding route after creating it

[dtolnay]

The symptom is that a route "fails" to create, then every subsequent terraform apply fails with RouteAlreadyExists.

CreateRoute was succeeding but the very next DescribeRouteTables was not listing the new route.

[catsby]

Looks great, thanks!
I pulled the branch locally and verified the acceptance tests still pass:

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSRoute_ -timeout 120m
=== RUN   TestAccAWSRoute_basic
--- PASS: TestAccAWSRoute_basic (31.49s)
=== RUN   TestAccAWSRoute_changeCidr
--- PASS: TestAccAWSRoute_changeCidr (46.93s)
=== RUN   TestAccAWSRoute_noopdiff
--- PASS: TestAccAWSRoute_noopdiff (94.28s)
=== RUN   TestAccAWSRoute_doesNotCrashWithVPCEndpoint
--- PASS: TestAccAWSRoute_doesNotCrashWithVPCEndpoint (33.82s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    206.535s
Test:

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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