Avoid a copy in RPC output

Split up HTTPReply into HTTPReply and HTTPReplyHeader, so that
the message data can be streamed directly.

Also removes a c_str(), which would have prevented binary
output with NUL characters in it.
Remove size limit in RPC client, keep it in server

The size limit makes a lot of sense for the server, as it never has to
accept very large data.

The client, however, can request arbitrary amounts of data with
`listtransactions` on a large wallet.

Fixes #4604.
Merge pull request #4640

733177e Remove size limit in RPC client, keep it in server (Wladimir J. van der Laan)
e17151a Avoid a copy in RPC output (Wladimir J. van der Laan)