Increase the default maximum context pool size

[ajcvickers]

Based on findings from the TechEmpower perf work. We probably want a very high default which can then be tuned down if the application wants to reduce maximum memory at the expense of speed.

[stevendarby]

1024 like the benchmarks?

[roji]

For completeness, we also discussed making this infinite by default (i.e. int.MaxValue). The issue with that is that if there's suddenly a spike due to some issue (e.g. a temporary network problem causes operations to block for a bit, and therefore many contexts get created), then the excess contexts will never be freed.

[davidfowl]

What's the strategy for clearing the pool?

[roji]

@davidfowl at the moment, the only time the pool is cleared is if its disposed; in the typical usage scenario, the pool is a singleton, so it never gets cleared (here's the code).

But note that unlike e.g. database connections, DbContexts are purely an in-memory thing, so keeping around e.g. 1024 for the entire application doesn't seem like a big problem. Are you thinking about this with dotnet/runtime#48567 in mind?

[ajcvickers]

We agreed in triage to make this 1024 so that we don't ship an unbounded pool by default.