lnrpc: set JS_STRING option for chan_id

This sets the `jstype` option to `JS_STRING` for all `chan_id` fields
in the proto rpc definition. `chan_id` is a 64 bit integer, which is
not natively supported by javascript's floating-point `number` with
only 52 bit precision. Nevertheless, by default protobuf will use the
`number` type for 64 bit integer fields in javascript, which can cause
loss of precision problems with `chan_id`. Explicitly setting the type
for javascript as a string will prevent these issues, and should not
interfere with its use as an identifier.