Introduce generic utility API to log message on invalid config entry.
Fix for #922.

Add more relevant error message when we supply invalid value for
defrag.trackers and defrag.hash-size
fix for #927.

Print an error message when the user supplies an invalid value for
detect-thread-ratio in the conf file.
fix for #926.

Supply meaningful error message when user supplies invalid value for
host.prealloc.
fix for #925.

Log sensible error message when the user doesn't supply a value for
stream.prealloc-sessions or when the values supplied in invalid and
the engine resorts to using a default.