Refactor script execution state machine in pgbench.
The doCustom() function had grown into quite a mess. Rewrite it, in a more
explicit state machine style, for readability.
This also fixes one minor bug: if a script consisted entirely of meta
commands, doCustom() never returned to the caller, so progress reports
with the -P option were not printed. I don't want to backpatch this
refactoring, and the bug is quite insignificant, so only commit this to
master, and leave the bug unfixed in back-branches.
Review and original bug report by Fabien Coelho.
Discussion: <alpine.DEB.2.20.
1607090850120.3412@sto>