cache results of classpath query because it takes forever

[eostermueller]

Selecting the "Step 2: Choose Workload" tab you wait several seconds for the SUT's classpath to be interrogated for snail4j's @load annotation. See screenshot below.

Until there is support of hot reload, this query needs to be cache so workload can quickly be changed.

Most of the changes need to be made here:
https://github.com/eostermueller/snail4j/blob/master/frontend/src/main/web/src/app/use-cases/use-cases.component.ts