Optimise dependencies instantiation

[nicusX]

Reorganise services, repositories and clients initialisation to avoid creating multiple instances and, better, only instantiating dependencies required by the specific endpoint

• HttpClient
• some DynamoClients (note: if #9 moves tablename at repository level, a single dynamo client instance may be shared across all repositories)
• IdentityRepository and IdentityService

[nicusX]

Measure actual memory consumption before and after optimising dependencies instantiation

[nicusX]

App classes are already selectively imported by handlers, so we are instantiating dependencies per lambda groups: Slack, Dropbox and Github