Improve performance in the TechEmpower composite score

[ajcvickers]

We have been running the industry standard TechEmpower benchmarks on .NET against a PostgreSQL database for several years. In the last couple of releases, we have significantly improved the Fortunes benchmark for both low-level data access and EF Core.

In the EF7 timeframe we plan to specifically target improvements to the TechEmpower composite score. This measures performance over a wider range of scenarios.

Most of the planned improvements are listed in #26797

This issue is focused on the work specific to EF Core. There will also be work at the lower-levels of the stack (e.g. ADO.NET/Runtime/Npgsql) and in the benchmark code itself.