Implement clear-all and get-all

clear-all can be implemented with empty byte-array as begin and 255 as the end parameter. This can be helpful where we want to wipe all the keys similar to FLUSHALL in Redis. Similar thing can be done to implement get-all using get-range to return all the keys in the database.

Reference: https://forums.foundationdb.org/t/how-to-clear-all-keys-in-foundationdb-using-java/351/2

This is not implemented as part of any official API but I thought it will be a handy wrapper. Feel free to close this if it's irrelevant.