make ThriftServer.h more portable
make ThriftServer.h more portable

Summary:
Older GCC versions (before 7.1) have a [bug](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67274) that requires explicit `this->` inside lambda functions in some cases.

Adding the explicit `this` fixes open-source HHVM builds (which include Thrift as a submodule) on older systems (Debian 8/9, Ubuntu 16.04).

Reviewed By: rhodo, bithree

Differential Revision: D20126933

fbshipit-source-id: ae088814fc351a24e46c2cc5f02a1a1670d4795c

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