[C#] Add locking methods to Functions

[TedHartMS]

Add the following to IFunctions and IAdvancedFunctions. Functions implementations that do not inherit from (Advanced)FunctionsBase must provide an implementation of these.

    // If true, FASTER calls Lock() and Unlock() where appropriate. Currently this is the 4 concurrent callback methods:
    // ConcurrentReader, ConcurrentWriter, ConcurrentDeleter (new in IAdvancedFunctions), and InPlaceUpdater.
    bool SupportsLocking { get; }

    // FunctionsBase illustrates the default implementation of these as an exclusive lock using a bit in RecordInfo
    void Lock(ref RecordInfo recordInfo, ref Key key, ref Value value, LockType lockType, ref long lockContext);
    bool Unlock(ref RecordInfo recordInfo, ref Key key, ref Value value, LockType lockType, long lockContext);

Add a new method ConcurrentDeleter to IAdvancedFunctions. This may return false for default deletion handling (setting tombstone, and setting object types to null), or may implement its own delete handling (which must set tombstone) and return true.

Add FASTER.benchmark: -z, --locking option to exercise