Use rocksdb instead of a new SCID5 (deps: chess960)

[benini]

Evaluate if rocksdb can be a valid alteranative to the binary, custom, SCID4 database format.
PRO:
-- less code to maintain
-- portable format
CONS:
-- external dependency, less easy for users to compile
-- very long compilation times

• SCID supports chess960
• Add a scripts/bench_codec.tcl
• Create a rocksdb branch
• Change CMakeFile.txt to download, compile and link rocksdb
• Create a simple, default CodecRocksDB and measure the performance (3 hrs)
• Try to improve rocksdb until it is as fast as SCID4 #36