fixing PQexec() calls

fixing several calls where we did not check the result status but only
the return value; the query may fail nonetheless
flush stderr after a log message appears

We had the problem that the log file appeared empty for a long time due
to file buffers. Thus we call fflush() after every log message so the
log file gets written out to disk quickly
fixing wait_connection_availability()

wait_connection_availability() did take at least 2 seconds per call in
the old incarnation. Now we may finish a call without any sleep at all
when the result is already ready at the time called
fixing repmgr repl_status columns

repmgr repl_status had the column time_lag which was documented to be
the time a standby is behind master. In fact it only works like this
when viewed on the standby and not on the master: there it only was the
time of the last status update. We dropped that column and replaced it
by a new column „communication_time_lag“ which is the content of the
repl_status column on the master. On the standby we contain the time of
the last update in shared mem though refer always to the correct time
nonetheless where repl_status is queried. We also added a new column,
„replication_time_lag“, which refers to the apply delay.