[c++] Add a new value_size() method to RmwContext for RCU operations

[matthewbrookes]

What:

• add a value_size() method that allows the user to compute the new Value size with respect to the old Value
• update tests etc to include the new method in RmwContext
• add a test to show the use of this method for a growing string

Why:

• supports RMW operations for a "growing" type such as a string, vector

Closes #136

[badrishc]

Thanks, this looks reasonable. Since it's a core change we will need a bit more testing so might take some time to merge. Have you done any performance regression tests with the benchmarks?

[matthewbrookes]

I haven't yet but I may get a chance over the weekend to run the RMW benchmark and compare

[matthewbrookes]

Hey, I ran the RMW benchmark against master and against this new branch. You can see my results in the following spreadsheet.

https://docs.google.com/spreadsheets/d/1NozTEkdRO1RHmjFBRhT3oxQExCy3TlXPTheyvYBpdhA/edit?usp=sharing

Whilst the results suggest an improvement in performance with these modifications, I think the difference isn't statistically significant and suggests that this change would not impact performance.