Because multiple threads can access the TrustStore to update/add/rm
certificates, introduce a mutex to protect the maps.

Because a lot of methods only access the maps in read-only, the
mutex is mutable. Moreover, because isAllowed will check the whole
chain, to avoid multiple lock/unlocks, the mutex is a recursive one.

Change-Id: Iec197221e2eefba4a7192f36f1a9a952f2533778
GitLab: #690