Introduce harcdoded timeouts for each RPC call

This patch adds a table with per-opcode timeouts. They were chosen in an
empiric, rather than scientific, way - see the comments in lib/rpc.py.

The patch also shows how custom timeouts can be used - call_test_delay
explicitly overrides the timeout with one computed from the delay
parameters.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>