Allow persistence to be explicitly selected

[rlf]

With the current extension methods, it would seem that the following would be selecting PostgreSql as persisitence:

services.AddEventFlow(o => o.UsePostgreSqlEventStore());

Unfortunately that is not the case, since the TryAdd in this line:
https://github.com/eventflow/EventFlow/blob/develop-v1/Source/EventFlow.PostgreSql/Extensions/EventFlowOptionsPostgresSqlEventStoreExtensions.cs#L34

Won't replace the TryAdd in this line:

EventFlow/Source/EventFlow/EventFlowOptions.cs

Line 185 in bb46378

serviceCollection.TryAddSingleton<IEventPersistence, InMemoryEventPersistence>();

A current workaround would be to either do:

services.TryAddTransient<IEventPersistence, PostgreSqlEventPersistence>();
services.AddEventFlow(o => {});

or

services.AddEventFlow(o => {});
services.AddTransient<IEventPersistence, PostgreSqlEventPersistence>();

But to me it seems that if you specifically use the UsePostgreSqlEventStore extension, it should be safe to add without try (i.e. overrule the sane defaults).

[rasmus]

Seems like a good change and possible a mistake on my part when I moved everything to the new extensions