Performance improvements

[rpopescu]

• add bool WireFormat::SkipString(CodedInputStream*) (used below);
• Client::Impl::ReceiveData() uses SkipString to skip over an unused string in the input, instead of creating a std::string, reading into it, then discarding it;
• Client::Impl::ReadBlock(...) reuses the std::string variables name and type for all columns, instead of allocating these two std::strings for each column;
• Client::Impl::Insert(...) builds the fields_section more efficiently, directly from the block, without creating and copying from the block into a std::vector<std::string>, and then writing to std::stringstream from it.