Rework the pgbench state machine code for clarity

This commit continues the code improvements started by commit
12788ae4.  With this commit, state machine transitions are better
contained in the routine that was called doCustom() and is now called
advanceConnectionState -- the resulting code is easier to reason about,
since there are no state changes occuring in the outer layer.

This change is prompted by future patches to add more features to
pgbench, which will need to effect some more surgery to this code.

Fabien's original had all the machine state changes inside one routine,
but I (Álvaro) thought that a subroutine to handle command execution is
more straightforward to review, so this commit does not match Fabien's
submission closely.  If something is broken, it's probably my fault.

Author: Fabien Coelho, Álvaro Herrera
Reviewed-by: Kirk Jamison
Discussion: https://postgr.es/m/alpine.DEB.2.21.1808111104320.1705@lancre