tp: parse appended trace data without copying it

Trace bytes arrive zero-copy into the tokenizer, and then Rpc::Parse
copies them straight back out because TraceProcessor needs to own what
it is given. For APPEND_TRACE_DATA that is a 32 MB memcpy per chunk,
which undoes the whole chain, on the wasm and httpd paths as much as
anywhere else.

Hand the parser the tokenizer's message instead: TraceBlob holds it for
as long as the bytes are needed, and the ring buffer moves onto another
buffer rather than recycling one still being read. Dispatching a message
now means handing over its bytes, so OnRpcMessage() takes an owned
message rather than a pointer and a length.

out/linux_clang_release/perfetto_integrationtests \
    --gtest_filter='*Remote*:*Stdiod*:*ServerUnix*'