address: add expiration time

[oleganza]

We should probably add a low-resolution expiration time to the addresses (introduced in #472): for instance, a 16-bit (2-byte) count of days starting from Jan 1, 2020. This gives 1-day resolution between years 2020 and 2199. The compatible software will automatically refuse to send money to addresses that expired allowing the recipients to avoid watching for old addresses and lower their storage requirements. 16 bits adds 3 characters to bech32 encoding of the address (because existing 64 bytes takes 102.4 5-bit characters which is rounded to 103 in present format).