Reland "Optimize PacketRouter/RTPSender interactions."

This reverts commit 66147e892dd6b7b1beaddbcab456a1ce28b2ad22.

Reason for revert: The culprit was https://webrtc-review.googlesource.com/c/src/+/133169.

Original change's description:
> Revert "Optimize PacketRouter/RTPSender interactions."
> 
> This reverts commit 6f129b3b7605dc69c8c188ca02d133250130570e.
> 
> Reason for revert: Speculative revert (some perf test are failing)
> 
> Original change's description:
> > Optimize PacketRouter/RTPSender interactions.
> > 
> > The legacy code-path uses a hashmap as cache in order to speed up
> > finding the right rtp module to send on. The new path should use that
> > as well.
> > In addition, there are checks that verify if an RTP module can send
> > padding, in some cases payload based. These result in a number of
> > calls to methods in RTPSender requiring its lock to be taken. This CL
> > introduces a combined SupportsPadding() check method which performs
> > all those checks in one go.
> > 
> > Bug: None
> > Change-Id: I2d18d0d6e7d8cfe92c81d08cef248a4daa7dcd4b
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144780
> > Reviewed-by: Åsa Persson <asapersson@webrtc.org>
> > Reviewed-by: Sebastian Jansson <srte@webrtc.org>
> > Commit-Queue: Erik Språng <sprang@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#28535}
> 
> TBR=asapersson@webrtc.org,sprang@webrtc.org,srte@webrtc.org
> 
> Change-Id: I8499dc0fd6e6d0b9fa7a0886c8754655e5589780
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: None
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145326
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#28552}

TBR=mbonadei@webrtc.org,asapersson@webrtc.org,sprang@webrtc.org,srte@webrtc.org

Change-Id: I3bff3ecb2b776e30f77c1884f6faa72b21788017
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145401
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28563}

modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -76,7 +76,8 @@ class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::ModuleRtpRtcp {
 
   int32_t DeregisterSendRtpHeaderExtension(RTPExtensionType type) override;
 
-  bool HasBweExtensions() const override;
+  bool SupportsPadding() const override;
+  bool SupportsRtxPayloadPadding() const override;
 
   // Get start timestamp.
   uint32_t StartTimestamp() const override;