Code to skip large single terms and report them

Since a term > approx 30k can cause merge-index to be unable to compact
due to an overflow bug, we can skip the killer term and issue an error
to the logs.
Improved check to look at size of entire posting

Was originally only checking the size of Term.  A sufficiently large
combination of bucket name, key name, etc could still cause an overflow
Removed `GeneratedObject` from log message

It was just too noisy and gross.  Ended up emitting > 32000 bytes to log
when it was there which was truncated by lager.
Changed sizing method

Originally used byte_length(to byte_size(term_to_binary(...)) which
causes the binary to be generated. Per Jon Meredith, switched to
erlang:external_size(...) which does the size estimation without
generating the binary
Merge branch 'cv-skip-largeterms' into 1.4