thundra_lambda_warmup_functionInstanceIdleTime property seems to be problematic

[ksrinath]

Compare the thundra_lambda_warmup_functionInstanceIdleTime property with the feature of "Multiple functions and their configurations can be specified with this prefix such as thundra_lambda_warmup_function1, thundra_lambda_warmup_function2 ..."

It seems that when the instance idle time property is specified, it's value (say 10x60x1000) is actually regarded as the name of a lambda function to be warmed up.

For example, logs show
2019-02-20 08:09:41,031 main INFO [thundra.lambda.warmup.WarmupHandler] Functions to warmup: {600000=WarmupFunctionInfo{alias='null', warmupStrategy=null, invocationCount=0, invocationData=null}, ServerlessSpringSample-PetStoreFunction-8GHLNQ7HFHDV=WarmupFunctionInfo{alias='null', warmupStrategy='stat-aware', invocationCount=4, invocationData=null}}

And this seems to lead to further errors effectively halting the processing even of the main lambdas to be warmed.