Keep only mvfst dependents in AsyncUDPSocket

[saubhik]

This PR modifies AsyncUDPSocket to only keep the functionality required by mvfst.

mvfst uses everything except the following API:

// The following may not be required (requires some changes in some functions to remove these completely)
bool AsyncUDPSocket::setZeroCopy(bool enable)
int AsyncUDPSocket::getZeroCopyFlags()
void AsyncUDPSocket::releaseZeroCopyBuf(uint32_t id)
bool AsyncUDPSocket::isZeroCopyMsg(FOLLY_MAYBE_UNUSED const cmsghdr& cmsg)
void AsyncUDPSocket::processZeroCopyMsg(FOLLY_MAYBE_UNUSED const cmsghdr& cmsg)
----

// The following are removed completely
void AsyncUDPSocket::dontFragment(bool df)
AsyncUDPSocket::TXTime AsyncUDPSocket::getTXTime()
bool AsyncUDPSocket::setTXTime(TXTime txTime)
bool AsyncUDPSocket::setRxZeroChksum6(FOLLY_MAYBE_UNUSED bool bVal)
bool AsyncUDPSocket::setTxZeroChksum6(FOLLY_MAYBE_UNUSED bool bVal)
void AsyncUDPSocket::setTrafficClass(int tclass)

How to add changes to folly as a mvfst dep?

• Update folly code in mvfst/_build/deps/folly/
• In mvfst's build_helper.sh, comment out the following lines in setup_folly():

# git clone https://github.com/facebook/folly.git "$FOLLY_DIR"
# synch_dependency_to_commit "$FOLLY_DIR" "$MVFST_ROOT_DIR/build/deps/github_hashes/facebook/folly-rev.txt"

• Run ./build_helper.sh
• Run cd _build/build && make test

[saubhik]

mvfst builds successfully with the reduced async UDP socket API. No failed tests:

100% tests passed, 0 tests failed out of 1473

Total Test time (real) =  30.49 sec