OPENDJ-1602 (CR-5566) New pluggable storage based backend
Copied the JE backend (without import) under the pluggable backend package.
Changed it to use the proof-of-concept backend interfaces instead of JE code.
BackendImpl.java:
Added interfaces from the backend proof-of-concept.
*.java:
Stripped all JE imports and replaced them with the backend proof-of-concept interfaces.
build.xml:
Code does not compile, so excluded "org.opends.server.backends.pluggable" from checkstyle checks and from compilation process.
StaticUtils.java:
Added bytesToHex(ByteSequence).