Split use of SerialSLRULock, creating SerialControlLock

Split use of SerialSLRULock, creating SerialControlLock

predicate.c has been using SerialSLRULock (the control lock for its SLRU
structure) to coordinate access to SerialControlData, another of its
numerous shared memory structures; this is unnecessary and confuses
further SLRU scalability work.  Create a separate LWLock to cover
SerialControlData.

Extracted from a larger patch from the same author, and some additional
changes by Álvaro.

Author: Dilip Kumar <dilip.kumar@enterprisedb.com>
Discussion: https://postgr.es/m/CAFiTN-vzDvNz=ExGXz6gdyjtzGixKSqs0mKHMmaQ8sOSEFZ33A@mail.gmail.com