http_request: reduce insn size for check_client_connection

http_request: reduce insn size for check_client_connection

Unlike constants and instance variables, class variable access
is not optimized in the mainline Ruby VM.  Use a constant
instead, to take advantage of inline constant caching.

This further reduces runtime instruction size by avoiding a
branch by allocating the Raindrops::TCP_Info object up front.

This reduces the method size by roughly 300 bytes on 64-bit.