Remove VCMEncoderDataBase and put remaining code into VideoStreamEncoder

This is a reland of
https://webrtc-review.googlesource.com/c/src/+/123920
Patch set 1 is identical to the previous CL, additional patch sets fix
the bug that was introduced and adds test coverage.

Since this "data base" only holds a single encoder instance it just
serves to confuse object ownership. Removing it and giving ownership
of generic encoder instance to VideoStreamEncoder.

This CL also removes VideoSender interface from video_coding_impl.h,
which is mostly a leftover from
https://webrtc-review.googlesource.com/c/src/+/123540

Bug: webrtc:10164
Change-Id: Ieaf23457d69af0d6356b70461112892b14760b19
Reviewed-on: https://webrtc-review.googlesource.com/c/124488
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26857}

modules/video_coding/BUILD.gn

@@ -102,8 +102,6 @@ rtc_static_library("video_coding") {
     "decoder_database.h",
     "decoding_state.cc",
     "decoding_state.h",
-    "encoder_database.cc",
-    "encoder_database.h",
     "fec_controller_default.cc",
     "fec_controller_default.h",
     "fec_rate_table.h",