[PLAT-3437] Use Longs in the maps.

Java performance has improved sufficiently since this was last benchmarked; this workaround is no longer faster.
[PLAT-3437] Create the error within a unit test, interrupting a read.
[PLAT-3437] Unit test for the string encoder.
[PLAT-3437] Run all Berkeley DB I/O in a different thread
[PLAT-3437] Factor the Berkeley DB I/O worker into a separate class.
[PLAT-3437] Refactor the BerkeleyDBBinaryDataStore to use an I/O worker thread.