Catalog Watch uses blocking queries incorrectly

Hello, I'm working on Consul and have seen several users face issues around how spring-cloud-consul interfaces with Consul so I've been trying to investigate.

I've noticed that spring-cloud-consul-discovery polls the Consul catalog very frequently with short-lived blocking queries (specifically the call to /v1/catalog/services?wait=2s&index=<index> at default 1s intervals).

The intention for blocking queries is that you wait on one connection which will hold until any catalog changes are detected by the agent. It is meant to be an efficient alternative to frequent polling.

It should be enough to make one call to /v1/catalog/services?index=<index> without the default deadline of wait=2s. In fact, removing the ?wait= would drastically reduce the network traffic to Consul clients with no impact to service discovery.

Start at spring-cloud-consul-discovery/src/main/java/org/springframework/cloud/consul/discovery/ConsulCatalogWatch.java around line 128 and inspect how the catalog request is built. Compare it with the Consul blocking-query behavior described in the issue. Done means the index-based catalog lookup no longer applies the short default wait while service discovery behavior remains unchanged.