[SPARK-13132][MLLIB] cache standardization param value in LogisticRegression

cache the value of the standardization Param in LogisticRegression, rather than re-fetching it from the ParamMap for every index and every optimization step in the quasi-newton optimizer

also, fix Param#toString to cache the stringified representation, rather than re-interpolating it on every call, so any other implementations that have similar repeated access patterns will see a benefit.

this change improves training times for one of my test sets from ~7m30s to ~4m30s

Author: Gary King <gary@idibon.com>

Closes #11027 from idigary/spark-13132-optimize-logistic-regression.