BUG-5280: fix snapshot accounting

BUG-5280: fix snapshot accounting

The following warning is emitted under testing:

2017-04-19 08:49:34,707 | WARN  | ... | AbstractClientHistory            | ... | Could not find aborting transaction member-2-datastore-operational-fe-0-txn-19-0

Which is indicating that we cannot find the open transaction
inside AbstractClientHistory.

The problem is mis-routed invocation when we are taking a snapshot:
instead of going directy to subclass doCreateSnapshot() which only
allocates the transaction, invoke takeSnapshot(), which actually does
the appropriate book-keeping.

Change-Id: I07473f381d3147a7fc7d355afede254a781a3094
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>