This bumps Mono to use mono/mono#17645 (which is the 2019-10 backport
of mono/mono#17628).

The big user-visible change is in regards to certificate validation, everything below are just
some minor adjustments to tests.

## SocketsHttpHandler

CoreFX uses a completely new `HttpClientHandler` implementation called `SocketsHttpHandler`,
which you can find at https://github.com/dotnet/corefx/tree/release/3.0/src/System.Net.Http/src/System/Net/Http/SocketsHttpHandler.

Since this is not based on the web stack anymore, it does not use any of the related APIs such
as `ServicePointManager` or `WebException`.

## Certificate Validation Changes

There is a new API called `HttpClientHandler.ServerCertificateCustomValidationCallback`.
- https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclienthandler.servercertificatecustomvalidationcallback?view=netframework-4.8
- https://github.com/dotnet/corefx/blob/c1778515a3bee34cc09c757b5563d0af0c8b1e99/src/System.Net.Http/src/System/Net/Http/HttpClientHandler.Unix.cs#L154
- https://github.com/dotnet/corefx/blob/c1778515a3bee34cc09c757b5563d0af0c8b1e99/src/System.Net.Http/src/System/Net/Http/HttpClientHandler.Windows.cs#L383

The `ServicePointManager.ServerCertificateValidationCallback` is no longer invoked and on
certificate validation failure, `AuthenticationException` (from `System.Security.Authentication`)
is thrown instead of `WebException`.

At the moment, the `NSUrlSessionHandler` still uses it's own validation callback and also still
throws `WebException` on failure; we should probably look into making this consistent with the
other handlers.

## Minor adjustments related to internal Mono APIs

* `HttpContent.SerializeToStreamAsync()` is now `protected` (changed from `protected internal`).
  - src/Foundation/NSUrlSessionHandler.cs: changed overload accordingly.
  - src/System.Net.Http/CFContentStream.cs: likewise.

* `HttpHeaders.GetKnownHeaderKind()` is an internal Mono API.
   There is a new internal API called `System.Net.Http.PlatformHelper.IsContentHeader(key)`
   which exists in both the old as well as the new implementation.
   The correct way of doing it with the CoreFX handler is
   `HeaderDescriptor.TryGet (key, out var descriptor) && descriptor.HeaderType == HttpHeaderType.Content`

## Minor adjustments to tests.

* `HttpClientHandler.MaxRequestContentBufferSize` is now longer supported, you can set it to
  any non-negative value, the getter will always return 0.
  See https://github.com/dotnet/corefx/blob/c1778515a3bee34cc09c757b5563d0af0c8b1e99/src/System.Net.Http/src/System/Net/Http/HttpClientHandler.Core.cs#L18.
  - tests/linker/ios/link sdk/HttpClientHandlerTest.cs: removed assertion from test.

* `HttpMessageInvoker.handler` is a `protected private` field - in the CoreFX handler, it is
  called `_handler` and `private`.  This is accessed via reflection by some of the tests, which are
  now using the new name.
  - tests/mmptest/src/MMPTest.cs: here
  - tests/mtouch/MTouch.cs: here

* tests/monotouch-test/System.Net.Http/MessageHandlers.cs:
  Adjust `RejectSslCertificatesServicePointManager` to reflect the certificate validation
  changes described above.
  - FIXME: There was an `Assert.Ignore()` related to `NSUrlSessionHandler` and macOS 10.10;
    I removed that to reenable the test because the description linked to an old issue in
    the private repo that was referenced by several "Merged" PR's, so it looked to me that
    this might have already been fixed - and I also didn't see why it would fail there.