Add a Lambda-warming route to Tilegarden

[KlaasH]

Whatever we decide to do for #701, we'll need a route in Tilegarden to hit for warming up Lambda instances. It's possible we'll want it to wait briefly before returning so that launching a handful of concurrent warming requests consistently results in a handful of concurrent instances.

[KlaasH]

As described on PR #712, this was implemented (https://github.com/azavea/pfb-network-connectivity/tree/feature/kjh/lambda-warming-endpoint) but ended up being more trouble than it's worth compared to the simpler scheduled warming approach. The implemented setup only involves a bit of code in the Lambda and a rule in CloudWatch, whereas an endpoint-based approach would require all of that plus more code in the Lambda and the back-end, plus database persistence.