When noting that a TStreamerInfo set has already been processed, we
must do it in 3 separates steps:
  - check it was seen before
  - read and process the set
  - record that the set as been seen.

The previous situation:
  - check and record that the set as been seen
  - read and process the set
led to a race condition if a second thread was checking the same
set before the second step was completed (in which case the
second thread was believing that the set was process and looking
for the result of the process (one of the StreamerInfo) but could
not find it.

We extend RConcurrentHashColl to have 2 new operations
  - standalone Hash calculation
  - standalone Find of hash
  - standalone Insert of hash.
and we use it to split the check and the recording as described
previously.

(this is an addendum to 95bf468438)

this helper is in the ROOT::Internal namespace. The hash used is sha256
through the ROOT::Internal::RSha256Hash class.
Even if this is technically a container, it's not in core/cont because of
dependency issues: we do not want core to depend thread and viceversa.