nostrdb: prune old contact lists

[jb55]

contact lists are by far the largest data items in nostrdb. We don't really need to store them all unless we want to do web of trust stuff in the future. For now we should at least prune older versions of contact lists.

# ndb stat dump of my damus db after 5-10 minutes of use
$ ndb stat

name                  count  key_bytes  value_bytes  total_bytes
---
dbs
---
note                  34833  278664     87122688     87401352
note_metadata         1057   33824      22996        56820
profile               4565   36520      2164336      2200856
note_index            34833  1393320    278664       1671984
profile_pubkey_index  4565   182600     36520        219120
nostrdb_metadata      1      8          8            16
profile_search        6900   441600     55200        496800
profile_last_fetch    4107   131424     32856        164280
total                 90861  2497960    89713268     92211228
-----
kinds
-----
profile               6067   48536      3145272      3193808
text                  14325  114600     6896360      7010960
contacts              3940   31520      70022592     70054112
dm                    4177   33416      1390880      1424296
repost                4099   32792      4514416      4547208
reaction              1996   15968      743240       759208
zap                   193    1544       285728       287272
list                  2      16         4960         4976
longform              14     112        105584       105696
status                7      56         1760         1816
other                 13     104        11896        12000

[alltheseas]

Adding as placeholder to 1.9 milestone @jb55 along with nostrdb updates