.NET Core 2 SDK is on the base Appveyor image now

This uses the new non-parallel-at-the-end I added to xUnit in https://github.com/xunit/xunit/pull/1411 This way we can run any global test that are other-test-unfriendly much more easily.

* Enable SqlDataRecord TVPs on netstandard1.3.
* Update netstandard2.0 SqlClient references to Preview 2.
* Skip TransactionScope tests for netcoreapp2.0.
- According to https://github.com/dotnet/corefx/issues/12534, ambient transaction enlistment is not included with .net core 2.0.
* Update references in Dapper.StrongName.
* Disable parallel test run to stabilize flaky type handler tests.
* Disable parallel test run by assembly attribute.
* Move Issue461 tests to TypeHandlerTests.
* Group type handler tests in a collection, re-enable parallelization.
- Being part of the same collection, type handler tests will run sequentially. All other tests can run in parallel.
* Move tests relying on query cache & type maps to a separate collection.

* Don't create the connection in Dispose: this avoids creating a SQLConnection simply to Dispose it.
* Run MySQL tests against the open MySqlConnection: 'connection' is a protected variable in the TestBase class that is a SQL Server connection, not a MySQL connection.
* Skip broken MySQL tests: as per issue #295, setting "AllowZeroDateTime=True" in the connection string causes an InvalidCastException in SqlMapper.

This adds a netstandard2.0 build to Dapper (and StrongName until v2),
adds a netstandard2.0 test lineup, restores most functionality (except
UDTs for SQL, e.g. .UdtTypeName isn't in netstandard2), and also breaks
things not actually implemented or implemented correctly in
netstandard2.0. Pushing this up so we can work through these with the
.NET teams.

Several things are still broken here (and fail tests):
- Parameter decimal values (not filed yet)
- DataTables as parameters
(https://github.com/dotnet/corefx/issues/19708)
- .GetSchemaTable() (not yet filed, implementation is completely missing
from SqlDataReader in CoreFX)

While the compile is clean, the above are runtime issues.
`SqlParameter.UdtTypeName` is a separate issue, that one just wasn't in
`netstandard2.0` at all...so we can't support UDTs there until it is.
See https://github.com/dotnet/corefx/issues/17126 for details.

More cleanup to do (removing Type column next), but getting to some very
usable output now.

This is based on BenchmarkDotNet (while preserving the legacy format
with minor improvements as well - legacy runs much faster). See #666 for
details. Not an ominus number at all.

Note: this code will get a bit simpler with BenchmarkDotNet categories,
see https://github.com/dotnet/BenchmarkDotNet/issues/248 for details.

It's indeed broken on their end. We should consider switching providers,
e.g. https://github.com/mysql-net/MySqlConnector/

* Progress on VS 2017 build tooling
* Finally, a working VS 2017 tooling build