lacre.repositories: Configure SQLAlchemy connection pooling

Provide 3 new configuration parameters in database section:

- max_connection_age --- number of seconds before an idle connection is
  "recycled", i.e. replaced with a new one;

- pool_size --- number of simultaneous connections kept in the pool;

- max_overflow --- maximum number of simultaneous connections we could make to
  the database.

Update sample config, including links to documentation.

gpg-mailgate.conf.sample

@@ -109,6 +109,18 @@ url = sqlite:///test.db
 # For other RDBMS backends, see:
 # https://docs.sqlalchemy.org/en/14/core/engines.html#database-urls
 
+# Number of seconds after which an idle connection is recycled.  This is
+# useful with MySQL servers.  For more information, see:
+# https://docs.sqlalchemy.org/en/14/core/engines.html#sqlalchemy.create_engine.params.pool_recycle
+#max_connection_age = 3600
+
+# Number of connections stored in the pool.
+#pool_size = 5
+
+# If the pool size is not enough for current traffic, some connections can be
+# made and closed after use, to avoid pool growth and connection rejections.
+#max_overflow = 10
+
 [enc_keymap]
 # You can find these by running the following command:
 #	gpg --list-keys --keyid-format long user@example.com

lacre/repositories.py

@@ -4,7 +4,7 @@ from sqlalchemy import create_engine, select, delete, and_, func
 from sqlalchemy.exc import OperationalError
 import logging
 
-from lacre.config import flag_enabled
+from lacre.config import flag_enabled, config_item_set, get_item
 from lacre._keyringcommon import KeyRing, KeyCache
 import lacre.dbschema as db
 
@@ -19,10 +19,27 @@ def connect(url):
     global _engine
 
     if not _engine:
-        _engine = create_engine(url)
+        config = _conn_config()
+        _engine = create_engine(url, **config)
 
     return _engine.connect()
 
+
+def _conn_config():
+    config = dict()
+
+    if config_item_set('database', 'max_connection_age'):
+        config['pool_recycle'] = get_item('database', 'max_connection_age')
+
+    if config_item_set('database', 'pool_size'):
+        config['pool_size'] = get_item('database', 'pool_size')
+
+    if config_item_set('database', 'max_overflow'):
+        config['max_overflow'] = get_item('database', 'max_overflow')
+
+    return config
+
+
 class IdentityRepository(KeyRing):
     def __init__(self, /, connection=None, db_url=None):
         self._identities = db.LACRE_IDENTITIES