src/lib/dhcpsrv/Makefile.am
    Added pgsql_exchange.cc and pgsql_exchange.h

src/lib/dhcpsrv/pgsql_exchange.h
src/lib/dhcpsrv/pgsql_exchange.cc
    New files, containng new base class PgSqlExchange from
    which was distilled from PgSqlLeaseExchange

src/lib/dhcpsrv/pgsql_lease_mgr.cc
    Refactored exchange classes to use new base class
    Moved PsqlBindArray into pgsql_exchange.*