Fix: Prevent race conditions when accessing slotmap in bankd

If a client connects to a worker, the worker will check if there is an
existing mapping between a reader and this client. If it exists,
slotmap_by_client() will return a pointer. If the mapping is deleted at
this time by the server, the worker uses a pointer to a mapping entry
that has just been freed.

To prevent this, the worker locks the slot map and calls the new
function slotmap_by_client_nolock(). After it has finished working with
the returned pointer, it unlocks the slot map again. A possible delete
by the main thread would be delayed.

Change-Id: I3464726f37beb7c47b4e1f00c018ffa4f3948906
---
M src/bankd/bankd_main.c
M src/slotmap.c
M src/slotmap.h
3 files changed, 21 insertions(+), 9 deletions(-)