Improve performance

[maxidorius]

mxisd was originally written as a proof of concept and some quick'n'dirty choices were made at the time to reach a full understanding of the ecosystem, at the cost of RAM & CPU usage, as well as caching. Now that mxisd is stable, it is time to address that.

Tasks:

• [x] Switch to Undertow instead of Spring Boot, reducing RAM & CPU usage
• [x] Only load Identity stores/Providers if enabled in config
• [x] Do not compute transnational requests more than once
  • [x] Identity: Bulk lookup
  • [x] AS: Transactions

[maxidorius]

Last task finished as of fbbafeb