server: Do not fail if client does not request sideband.

server: Do not fail if client does not request sideband.

Currently we insist that clients request the sideband feature during
the initial handshake. This check was introduced long ago to detect
incompatible 0.4.x clients and fail the connection early and gracefully
for those clients. These days the check is no longer needed and can
be removed.

para_server still announces the sideband feature and accepts the
sideband request, although it makes no difference any more whether
or not the client requests the feature. Therefore this change has no
impact on compatibility with 0.5.x clients.