FLASH-562: Fix race condition of batch command handling

[zanmato1984]

1. Make grpc server context as const as possible to not worry about thread safe;
2. Wrap single command handling lambda of batch command handler with a function to avoid lambda capture pitfalls;
3. Pre-allocate space for the vector of batch commands return values to avoid dangling element reference caused by buffer reallocation and element movement.

[zanmato1984]

This pre-allocation is a must, otherwise the following rets.back() reference will become dangling after buffer re-allocation when inserting more elements.

[zanmato1984]

The pitfall here is that, lambda will capture resp itself by reference (i.e. the address of variable resp, which is actually a local variable within the scope of for loop and become invalid when jumping out of for loop), rather than the reference to *resp which is the truly reference we want to capture.

The same rule applies to rets.back() as well, lambda captures rets rather than rets.back(), resulting in all threads calling rets.back() when running, i.e. writing results all to the last element.

Fix by evaluating *resp and rets.back() instantly as arguments of an extra function (handleCommandJob), and capture them within the function.

[windtalker]

LGTM