triedb/pathdb: use binary.append to eliminate the tmp scratch slice

[jsvisa]

binary.AppendUvarint offers better performance than using append directly, because it avoids unnecessary memory allocation and copying.

In our case, it can increase the performance by +35.8% for the blockWriter.append function:

benchmark                        old ns/op     new ns/op     delta
BenchmarkBlockWriterAppend-8     5.97          3.83          -35.80%