Fix syslogger to not lose log coherency under high load.

Fix syslogger to not lose log coherency under high load.

The original coding of the syslogger had an arbitrary limit of 20 large
messages concurrently in progress, after which it would just punt and dump
message fragments to the output file separately.  Our ambitions are a bit
higher than that now, so allow the data structure to expand as necessary.

Reported and patched by Andrew Dunstan; some editing by Tom