thrift: allow reusing struct fields

[achille-roussel]

This is a small change to allow reusing inner pointer fields of struct values in the thrift decoder. The existing implementation would always zero out the struct value before decoding it, which means pointers are set to nil and whatever object they were referencing is lost.

Most applications won't need this optimization and can allocate new objects to decode values into (they will have the zero value then). Applications that do need this optimization are aware that they must zero-out the objects they reuse across decoding operations, which makes it a meaningful change.

[kevinburkesegment]

Applications that do need this optimization are aware that they must zero-out the objects they reuse

Where would that be noted? I guess I don't see any docs on Decoder or NewDecoder which seem like the obvious places to look.

[kevinburkesegment]

ok LGTM

[achille-roussel]

Thanks for the review!