Implement #1641 rpc call public functions as read-only

[psq]

In starting to look into implementing #1981, I stumbled upon the implementation of call-read and thought maybe there was a way to call the same function in a read only fashion if it was made public instead of read-only.

And turns out, that the implementation was quite simple by changing a parameter and checking whether the code made any writes, and fail if it did.

#1981 still forthcoming.

Not consensus altering, so can be merged to master directly.

[psq]

couldn't link to #1641, but this will fix

[jcnelson]

We will want to make sure that this works with multiple concurrent write operations. If this creates savepoints, and unwinds savepoints, we'll need to make sure that one RPC call terminating does not (a) error out or (b) cause other in-flight RPCs to error out due to savepoint unwinding. Consider using a HTTP load tester to simulate lots of concurrent requests. We should also add a test to make sure that the order in which savepoints are unwound does not cause other savepoints to be clobbered.

[psq]

closing for now, will reopen when ready