Added support for new RPC support using HTTP POST. RPC's
arguments and reply are Gob encoded and sent as HTTP
request/response body.
This patch also removes Go RPC based implementation.
This change disables the non-constant-time implementations of P-384 and P-521.
As a consequence a client using just these curves cannot connect to the server.
This should be no real issues because (all) clients at least support P-256.
Further this change also rejects ECDSA private keys of P-384 and P-521.
While non-constant-time implementations for the ECDHE exchange don't expose an
obvious vulnerability, using P-384 or P-521 keys for the ECDSA signature may allow
pratical timing attacks.
Fixes#5844
