Revision 1129597

More efficient term size calculation

Unlike byte_size(term_to_binary(Term)), the BIF erlang:external_size/1 doesn't
do the serialization step, it only calculates the maximum external size for
any term, which is more efficient (faster and avoids the garbage generation).

With the test couch_http_bulk_writes.sh at [1], using 20 writers and batches
of 100 1Kb documents, it's possible to write about 1 400 000 documents with
this patch instead of about 1 300 000.

[1] https://github.com/fdmanana/basho_bench_couch