Remove DSR (Direct Server Return) from mvfst, ti/bigcache, and proxygen

Summary:
This commit completely removes the DSR (Direct Server Return) feature from the codebase. DSR was a zero-copy QUIC packet transmission mechanism that allowed backend servers to directly packetize and send data to clients, bypassing the transport layer.

Components removed:

**QUIC (fbcode/quic/)**
- Deleted quic/dsr/ directory with all DSR implementation files
- Removed BufferMeta and WriteBufferMeta from StreamData
- Removed writeBufMeta() and setDSRPacketizationRequestSender() from QuicSocket
- Removed DSR tracking from stream state (dsrSender, writeBufMeta, retransmission/loss buffers)
- Removed DSR packet tracking fields (isDSRPacket, nonDsrPacketSequenceNumber)
- Removed DSR logic from loss detection, flow control, packet scheduling
- Removed TperfDSRSender tool and DSR test infrastructure

**ti/bigcache**
- Deleted ti/bigcache/dsr/ directory (16 files: Cookie, PacketizationContext, SocketProvider, etc.)
- Removed ~1000 lines of DSR code from BigCacheCommandHandler (header/impl/inline)
- Removed XSK (AF_XDP) initialization and container management from MCBigCache.cpp
- Removed DSR RPC methods: async_eb_bigcacheGetDSR, async_eb_establishPacketizationSink, async_eb_release
- Removed getDSRBigcacheValueAsync(), createDSRDelegationRequest() from McrouterCache
- Removed DSR-specific stats namespace (dsrstats) and metrics
- Cleaned up BUCK dependencies removing quic/dsr and XSK references

**proxygen HTTP infrastructure**
- Removed allowDSR() virtual method from HTTPMessageFilter base class and 50+ filter implementations
- Removed setDSRRequestSender() interface from HTTPTransaction::Transport and HTTPSink
- Removed DSRRequestSender class and sendHeadersWithDelegate() from HTTPTransaction
- Removed BufferMeta struct and all related methods from HTTPTransaction (getBufferMeta, clearBufferMeta, etc.)
- Removed bufMeta_ member and references from HQSession::HQStreamTransportBase
- Removed isDelegated_ flag and delegated transaction checks from HTTPTransaction
- Removed cache DSR methods: canDSR(), performDSRDelegateAndScheduleCallbacks(), onDelegateSuccess(), onDelegateMiss()
- Removed DSR delegation from HTTPResponseCache and McrouterCache

**Test infrastructure fixes**
- Removed quic/dsr/test:mocks dependency from proxygen session test TARGETS
- Fixed PTM test to remove nonDsrPacketSequenceNumber field references
- Cleaned up MockQuicSocketDriver to remove DSR mock infrastructure (BufferMeta, writeBufMeta)
- Removed sendHeadersWithDelegate() from HTTPSessionMocks test helpers
- Commented out DSR-specific tests in HQDownstreamSessionTest (DelegateResponse, DelegateResponseError)
- Removed MockQuicDSRRequestSender from HQSessionMocks
- Disabled BigcacheSRPacketizer build targets (depends on removed quic/dsr infrastructure)
- Fixed WebTransportFilter, CdnFilters, and other components after removing DSR infrastructure

The codebase now exclusively uses standard QUIC stream writes through pendingWrites buffers.

Reviewed By: kvtsoy

Differential Revision: D86992558

fbshipit-source-id: a3814eaf735accdce989c45da8101aac8e8c831f

quic/logging/FileQLogger.cpp

@@ -257,7 +257,6 @@ void FileQLogger::addTransportSummary(const TransportSummaryArgs& args) {
           args.finalPacketLossTimeReorderingThreshDividend,
           args.usedZeroRtt,
           args.quicVersion,
-          args.dsrPacketCount,
           args.initialPacketsReceived,
           args.uniqueInitialCryptoFramesReceived,
           args.timeUntilLastInitialCryptoFrameReceived,