JBLOGGING-100 conditional string conversion

`JBossLogManagerLogger#doLog` always converts the message to a String
even if the level isn't loggable. For example `LogAuditProvider#audit`
does trace logging which ends up calling `AuditEvent#toString` even if
trace logging isn't enabled. In our case this results in a lot of
allocations outside of TLABs.

Issue: JBLOGGING-100
https://issues.jboss.org/browse/JBLOGGING-100