[#131] Update All Mutable Stores When Setting or Removing Values

[yakovmanshin]

[Closes #131]

• Previously, only the first mutable store was updated when FeatureFlagResolver received a call to set or remove a value
• In this update, the behavior changes: Now all matching stores (i.e. mutable or synchronous mutable) are updated accordingly
• If one of the stores throws an error in the process, this error is caught and kept by the resolver; this is to avoid incomplete operations where an error in a single store prevents updates to the others
• After the remaining stores are updated; the last thrown error is re-thrown by the resolver