KeyUtil: Initialize ENCODER_IMPL to StandardKeyEncoder

Also move StandardKeyEncoder to the client package, so that the class
is included in GWT module. Consequently, we don't need to initialize
ENCODER_IMPL to StandardKeyEncoder neither in Database no in
NoSqlDatabase classes.

Change-Id: I4b1b8de6949a6cc94bd004e2f983bb50b0a3270a
7 files changed