Add keys query scheduler

[Anderas]

Fixes #1668 and partially resolves #1670

Add MXKeysQueryScheduler used with crypto V2, which serves similar purpose as MXDeviceList serves in legacy crypto, albeit only focusing on optimizing keys/query requests.

Specifically it will:

• ensure that only one keys/query request is in-flight at any given time
• all further requests are aggregated, and if they contain any new users, will be executed right after the first one finishes

These optimizations should ensure that the application does not make unnecessary amout of queries, as we currently call the downloadKeys from far too many places. Note that I have previously tried to solve the current/next task in more generic way, but was missing the additional need to aggregate further users. So for now I am adding a case specific MXKeysQueryScheduler which may later evolve to more generic MXSerialTaskScheduler

In general the application should never query keys directly, except for very few cases (starting new room, refreshing cross signing), but this is not unfortunatelly the case at the moment. To start moving in the right direction I split out downloadKeysIfNecessary and a deprecated reloadKeys instead of passing forceDownload parameter. This should make it clearer which API to use.

[gileluard]

lgtm 👍 It seems your updates are causing some regressions in the CI though and MXKeysQuerySchedulerUnitTests.test_executeEachAliceQuerySeparately is failing

[Anderas]

lgtm 👍 It seems your updates are causing some regressions in the CI though and MXKeysQuerySchedulerUnitTests.test_executeEachAliceQuerySeparately is failing

Thanks, I actually forgot to add them to unit test plan as well, and missed the failure.