Deal better with database connection loss (Issue #6)

    * Wrap all database operations (including reads) in an SQLAlchemy transaction
      per http://docs.sqlalchemy.org/en/latest/orm/session_basics.html#when-do-i-construct-a-session-when-do-i-commit-it-and-when-do-i-close-it
    * Add configuration parameters to set the pool_recycle and pool_pre_ping engine parameters
      defaults are to recycle in 30 minutes and to always ping

sql.py

@@ -1,6 +1,7 @@
 # vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4
 
 import logging
+from contextlib import contextmanager
 from jsonpickle import encode, decode
 from typing import Any
 from sqlalchemy import (
@@ -35,27 +36,38 @@ def __init__(self, session, clazz):
         self.session = session
         self.clazz = clazz
 
+    @contextmanager
+    def _session_op(self):
+        try:
+            yield self.session
+            self.session.commit()
+        except:
+            self.session.rollback()
+            raise
+
     def get(self, key: str) -> Any:
         try:
-            return self.session.query(
-                self.clazz).filter(self.clazz._key == key).one().value
+            with self._session_op() as session:
+                result = session.query(self.clazz).filter(self.clazz._key == key).one().value
         except NoResultFound:
             raise KeyError("%s doesn't exists." % key)
+        return result
 
     def remove(self, key: str):
         try:
-            self.session.query(
-                self.clazz).filter(self.clazz._key == key).delete()
-            self.session.commit()
+            with self._session_op() as session:
+                session.query(self.clazz).filter(self.clazz._key == key).delete()
         except NoResultFound:
             raise KeyError("%s doesn't exists." % key)
 
     def set(self, key: str, value: Any) -> None:
-        self.session.merge(self.clazz(key, value))
-        self.session.commit()
+        with self._session_op() as session:
+            session.merge(self.clazz(key, value))
 
     def len(self):
-        return self.session.query(self.clazz).count()
+        with self._session_op() as session:
+            length = session.query(self.clazz).count()
+        return length
 
     def keys(self):
         return (kv.key for kv in self.session.query(self.clazz).all())
@@ -89,6 +101,8 @@ def __init__(self, bot_config):
         else:
             self._engine = create_engine(
                 config[DATA_URL_ENTRY],
+                pool_recycle=config.get('connection_recycle', 1800),
+                pool_pre_ping=config.get('connection_ping', True),
                 echo=bot_config.BOT_LOG_LEVEL == logging.DEBUG)
         self._metadata = MetaData()
         self._sessionmaker = sessionmaker()