feat: implement atomic operations

[yuryu]

What type of PR is this?
/kind feat

What this PR does / Why we need it:
Add the following gRPC methods

• CompareAndSwap
• CompareAndSwapGreaterInt
• CompareAndSwapLessInt
• AtomicAddInt
• AtomicSubInt
• AtomicInc
• AtomicDec

Which issue(s) this PR fixes:

Closes #134

Special notes for your reviewer:

[yuryu]

I'm not sure if these method names are good or do we want to clarify that they are for properties?
For example we can prepend them with Property

[yuryu]

I think we should name these methods as IntProperty*. I'll send a commit later today.

[yuryu]

So I did the rename, also changed some of the messages so that they would be easier to use.

• CompareAndSwap is now CompareAndSwapProperty
• all other methods were renamed to ...Int because it is pretty clear only Properties can hold structured data.

PTAL, thanks.

[ghost]

I have added a few suggestions to keep the function names less cryptic and more streamlined. I do not feel strongly about them, except for the AtomicIncInt and AtomicDecInt, which is really confusing when reading the implementation with function names such as doAtomicIncDec (maybe rename to doAtomicOperation).

[yuryu]

Did the renames according to our discussion.
Now

• rpc CompareAndSwap(CompareAndSwapRequest) returns (CompareAndSwapResponse) {}
• rpc CompareAndSwapGreaterInt(AtomicIntRequest) returns (AtomicIntResponse) {}
• rpc CompareAndSwapLessInt(AtomicIntRequest) returns (AtomicIntResponse) {}
• rpc AtomicAddInt(AtomicIntRequest) returns (AtomicIntResponse) {}
• rpc AtomicSubInt(AtomicIntRequest) returns (AtomicIntResponse) {}
• rpc AtomicInc(AtomicIncRequest) returns (AtomicIntResponse) {}
• rpc AtomicDec(AtomicIncRequest) returns (AtomicIntResponse) {}

The rationale is that inc/dec is always performed on integers.