HTTP/3: request more client body bytes.

HTTP/3: request more client body bytes.

Previously the request body DATA frame header was read by one byte because
filters were called only when the requested number of bytes were read.  Now,
after 08ff2e10ae92 (1.19.2), filters are called after each read.  More bytes
can be read at once, which simplifies and optimizes the code.

This also reduces diff with the default branch.