Improve super-large .snmprec file handling performance

[etingof]

This issue is to improve SNMP command responder scalability when serving very large files (seeks are slow). Compressing such files (which has been implemented) adds more time because seek in compressed data stream is not readily supported.

May be a way to go is to split large .snmprec files into smaller ones, and put them all into a directory which would be used for data source addressing, just like .snmpec files.