Allows users of the RPC CLI to set the `min_final_cltv_expiry_delta`
described in BOLT's 11, 7, and 2 by setting the `cltv_expiry` flag when
calling either of the `addinvoice` or `addholdinvoice` RPC's from
`lncli`.
Add additional comment to `lnclic addinvoice`'s `--private` flag warning
about including a large number of private channel hints when using zero
ammounts.
Also added a comment to the `private` field in the `Invoice` proto
message.
* pass amt and amt_msat to rpc, letting server give an error if both
are present
* take amt from an extra argument if neither amt nor amt_ms are
present
implementation analogous to 298f35cdfb,
which is for addholdinvoice.
