Werk #5707: Windows: allow eventlog monitoring from multiple hosts

Until now, Windows eventlogs could be monitored only from one host (Check_MK site). Attempting to contact one single Windows agent from multiple hosts lead to lost eventlog entries as all hosts shared one common state file for storing the offsets of so far read eventlog entries.

Now the offsets are store in host-IP-specific state files, allowing hosts with different IP addresses to monitor one single Windows system without loosing eventlog entries. Note: multiple Check_MK sites running under the one and same IP address will still suffer from lost eventlog entries as the offsets are stored per IP address.

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