THRIFT-2429

[djwatson]

Original diff summary:

Add "terse_writes" c++ gen option to thrift compiler.

Summary:
Add "terse_writes" c++ generator option to thrift compiler, to
suppress writing non-optional/non-required fields if they haven't
changed from the default value.

 - This is default-off, whitelist-only.
   It may be whitelisted per-.thrift, or per struct.

 - On a Unicorn loadgen benchmark appeared to save roughly 2/3rds of
   the SearchRequest deserialization cost (fields are only sparsely set).

 - This will not affect embedded structs, or strings with non-""
   default values, since we only want this change to do inexpensive
   integer-like comparisons.

 - Also, the user is advised that:
   - default values for whitelisted structs shouldn't be changed.
     (or, if they are, upgraded to optional/required)
   - if one re-read()s into an old struct, one should __clear() it first.
     however, this is not a new problem for anybody with optional fields.

This diff introduces "safe" mode for `terse_writes` in order to
prevent us from producing stupid bugs in PHP (and Python) land.

If we don't require explicit default value to be set, we have to
make PHP / Python logic to correctly understand `null` / `None` and
treat them as default field values.

davejwatson:

It sounds like the proposal in THRIFT-2429 is to make this the default for optional fields as well, with no compiler flag, which is just minor changes to this diff.

[asfbot]

Build triggered. Test FAILed.

[jeking3]

This pull request is being closed due to lack of activity.
You can re-open it after rebasing on the current master if you would like it to be reconsidered.