Fixed compilation problems in val.cc.
Added batched_replace function in val.hpp.
Replaced variadic table_t::replace function.
Petty reshuffling of code in writes.hpp.
Added datum_stream_t::next_batch with next_batch_impl having a default implementation.
Implemented array_datum_stream_t::next_batch_impl.
Made inserts use batched stuff and limited array stream batches to 100.