use readahead thread for file reads

[planthaber]

Tied to add a read ahead thread that reads future elements before it is requested

Was able to play a minio remote log continuously with 0.3 speed (via VPN)

Todo:

• Test if queue size matters
• Check if other calls are still correct (e.g. eof reports end of file also when buffer (queue) not empty )
• Compare performance without mmap