Expose WebAppMain#DEFAULT_RING_BUFFER_SIZE actual value

For Evergreen, we want to detect if already too many logs were generated before the Evergreen Jenkins Plugin even had time to start up.

The rationale being: if there are already 256 logs when plugin startup state is attained, then there is something (very) wrong per-se.

The goal is to be able to do something like:

if (Jenkins.logRecords.size() >= WebAppMain.getDefaultRingBufferSize()) {
    LOGGER.log(Level.SEVERE,
               "The Jenkins logs buffer is already full. " +
                       "Some logs might have been missed, and so many logs shows something probably very wrong! (max={0})",
               maxNumberOfLogs);
}

Currently, as an intermediate shim, I'm using reflection. In addition to being dirty, this is likely to become even more an issue to support Java 9 when using such hacks is being made harder and harder.