Securejoin: store bobstate in database instead of context

The state bob needs to maintain during a secure-join process when
exchanging messages used to be stored on the context.  This means if
the process was killed this state was lost and the securejoin process
would fail.  Moving this state into the database should help this.

This still only allows a single securejoin process at a time, this may
be relaxed in the future.  For now any previous securejoin process
that was running is killed if a new one is started (this was already
the case).

This can remove some of the complexity around BobState handling: since
the state is in the database we can already make state interactions
transactional and correct.  We no longer need the mutex around the
state handling.  This means the BobStateHandle construct that was
handling the interactions between always having a valid state and
handling the mutex is no longer needed, resulting in some nice
simplifications.

Part of #2777.

src/sql/tables.sql

@@ -168,6 +168,14 @@ CREATE TABLE tokens (
     timestamp INTEGER DEFAULT 0
 );
 
+-- The currently running securejoin protocols, joiner-side.
+-- CREATE TABLE bobstate (
+--     id INTEGER PRIMARY KEY AUTOINCREMENT,
+--     invite TEXT NOT NULL,
+--     next_step INTEGER NOT NULL,
+--     chat_id INTEGER NOT NULL
+-- );
+
 CREATE TABLE locations (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     latitude REAL DEFAULT 0.0,