add new db table for did cache

src/millipds/database.py

@@ -216,6 +216,19 @@ def _init_tables(self):
 			"""
 		)
 
+		# we cache failures too, represented as a null doc (with shorter TTL)
+		# timestamps are unix timestamp ints, in seconds
+		self.con.execute(
+			"""
+			CREATE TABLE did_cache(
+				did TEXT PRIMARY KEY NOT NULL,
+				doc TEXT,
+				created_at INTEGER NOT NULL,
+				expires_at INTEGER NOT NULL,
+			)
+			"""
+		)
+
 	def update_config(
 		self,
 		pds_pfx: Optional[str] = None,

src/millipds/static_config.py

@@ -24,3 +24,6 @@
 	100  # might want to tweak this upwards on a very active PDS
 )
 # NB: each firehose event can be up to ~1MB, but on average they're much smaller
+
+DID_TTL = 60 * 60 * 24  # 1 day
+DID_ERROR_TTL = 60 * 5  # 5 mins