Call closures directly where safe

src/core/ext/transport/chttp2/transport/chttp2_transport.c

src/core/lib/iomgr/closure.c

src/core/lib/iomgr/closure.h

src/core/lib/iomgr/exec_ctx.c

src/core/lib/iomgr/tcp_posix.c

src/core/lib/surface/server.c