[Performance] Recover cryptographic material of issuers in a concurrent way

When fetching the cryptographic material of the issuer, the trust evaluation mechanism try to invoke each configured trust handler implementation in a serial way.
However, each individual trust handler implementation might be somewhat slow in producing the trust material of an issuer, either due to slow computations mechanism (unlikely) or due to I/O bound operation like making a network call or finding/updating data in a DB (more likely).
Since trust handlers are called serially, this could possibly lead to a very slow global trust evaluation mechanism, especially if multiple trust handlers are configured.
One possible solution to that problem is to evaluate configured trust handlers in a concurrent way so that the impact of blocking I/O calls to the global trust evaluation mechanism is minimized.

I want to stress that this is not a critical issue right now, but it would be a nice to have (eventually), especially if we have as goal the support as many trust evaluation mechanism as possible.