Add configuration for tunable dataloss sensitivity

There are many cases in HBase when the process detects that some data is "not quite right". We often have two choices: (a) throw up our hands and exit, or (b) log a warning and push through the issue, potentially losing data. I think some users are very sensitive to dataloss and would prefer that the system become unavailable rather than continue with potentially lost data. Other users just want the system to stay up, and if they lose a log segment it's fine.