Update the PrivacyMapDB interface methods to take contexts (both the
methods themselves and the call-back params) and then ensure all
implementations are updated and all call-sites pass contexts through
correctly.
This commit expands the RealToPseudo methods to take in a privacy map db
reader. This allows the methods to check if the privacy map db already
contains an entry for a "real" string before generating a new one.
For now, only an empty PrivacyMapReader is ever provided to the
RealToPseudo call. This will be changed in the following commit.