pw_rpc: Expand server-side packet processing

- Handle various error cases with incoming packets.
- Dynamically assign a channel when a packet's ID doesn't exist.
- Reserve space in the response buffer for packet "header" fields.

Change-Id: Ibdce99c8ff1d37aa46bb4e400a4d8f8e646a8ac7

pw_protobuf/encoder.cc

diff --git a/pw_protobuf/encoder.cc b/pw_protobuf/encoder.cc
index 05ce88a..c20b093 100644
--- a/pw_protobuf/encoder.cc
+++ b/pw_protobuf/encoder.cc
@@ -56,7 +56,10 @@
     return encode_status_;
   }
 
-  memcpy(cursor_, ptr, size);
+  // Memmove the value into place as it's possible that it shares the encode
+  // buffer on a memory-constrained system.
+  std::memmove(cursor_, ptr, size);
+
   cursor_ += size;
   return Status::OK;
 }
@@ -167,7 +170,7 @@
       to_copy = end - read_cursor;
     }
 
-    memmove(write_cursor, read_cursor, to_copy);
+    std::memmove(write_cursor, read_cursor, to_copy);
     write_cursor += to_copy;
     read_cursor += to_copy;