Archive Command plugin is incompatible with PG 9.6 and lower

[cuprumtan]

PG 9.6 and lower does not have function called 'pg_current_wal_lsn()':

Bootstrap execution have exited with an error: {'S': 'ERROR', 'V': 'ERROR', 'C': '42883', 'M': 'function pg_current_wal_lsn() does not exist', 'H': 'No function matches the given name and argument types. You might need to add explicit type casts.', 'P': '3182', 'F': 'parse_func.c', 'L': '523', 'R': 'ParseFuncOrColumn'}

Use instead pg_current_xlog_location().

[cuprumtan]

Fixed bootstrap function.

TODO: edit queries in AC plugin