Add support for shared transaction in libprivilege-control DB.

Add support for shared transaction in libprivilege-control DB.

[Issue#]       SSDWSSP-722
[Feature]      No redundant rules updating during shared read
               transactions.
[Cause]        Rules in the DB are unnecessarily updated during shared
               read transactions.
[Solution]     Rules in the DB are updated only if the current
               transaction is not a shared read transaction.
[Verification] Although no additional tests were created, all previous tests
               should pass.

Change-Id: I6738add767cf1b666680a956ad19572a99d693e5