rgw: increase beast parse buffer size to 64k

rgw: increase beast parse buffer size to 64k

the flat_buffer starts very small and rarely grew above 512 bytes when
reading headers. this buffer's capacity() is what determines the size of
reads when when recv_body() calls boost::beast::http::async_read_some().
raising this buffer size to a static 64k greatly reduces the number of
socket reads

Reported-by: Marcus Watts <mwatts@redhat.com>
Signed-off-by: Casey Bodley <cbodley@redhat.com>