Add definable timeout for SyncStorage APIs

Add definable timeout for SyncStorage APIs

Add a new synchronous API function 'SyncStorage::setOperationTimeout()',
which can be used to set given timeout value for synchronous SDL
instance. If SDL database backend (Redis) is not available, hanging SDL
write, read or delete operation is terminated and an exception is raised
after timeout time has been elapsed. By default if timeout value hasn't
been set, hanging SDL synchronous operation is blocked indefinitely if
Redis is not available.

Following additional improvements:
 - Improved AsyncConnectionImpl UT coverage
 - Fixed member variable order in Hiredis*EpollAdapter-classes.
   Hiredis*System must be first member variable because its functions
   are called via engine event handler. Thus it must be before engine.
 - Redis disconnected debug log is now written also in success status
   (SDL initiated) disconnection.

Issue-ID: RIC-226

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I46e5e9738c25edb1c0d068dff739e765ca11096a