Added ngx_http_set_complex_value_size_slot().

Added ngx_http_set_complex_value_size_slot().

If a complex value is expected to be of type size_t, and the compiled
value is constant, the constant size_t value is remembered at compile
time.

The value is accessed through ngx_http_complex_value_size() which
either returns the remembered constant or evaluates the expression
and parses it as size_t.