Make live route writes cheaper so the prefix handler keeps up (closes #96)

The live route tables lose their random UUID key; the row id the
prefix-length sample draws is declared as the integer key. The prefix
tables are keyed by prefix and keep only first_seen; a batch inserts only
prefixes not stored yet. A route update sets only the columns that
change. Each kept index names the queries using it. Every write goes
through one connection kept for writing, with a 1 GiB page cache; reading
connections get 16 MiB, heap limits rise to 1.5 and 2 GiB (README
"Memory"). Batch writes go in key order. A benchmark run by hand writes
250,000 updates on 5 million routes faster than the feed's 10,000 a
second; a live run follows.

Model: opus-5-5

internal/database/counts_internal_test.go

@@ -29,7 +29,6 @@ func mkV4Route(t *testing.T, prefix string, asn int, ts time.Time) *LiveRoute {
 	}
 
 	return &LiveRoute{
-		ID:          uuid.New(),
 		Prefix:      prefix,
 		MaskLength:  maskLength,
 		IPVersion:   ipVersionV4,
@@ -44,7 +43,6 @@ func mkV4Route(t *testing.T, prefix string, asn int, ts time.Time) *LiveRoute {
 // mkV6Route builds an IPv6 live route.
 func mkV6Route(prefix string, asn int, ts time.Time) *LiveRoute {
 	return &LiveRoute{
-		ID:          uuid.New(),
 		Prefix:      prefix,
 		MaskLength:  32,
 		IPVersion:   ipVersionV6,