Finalize Firecracker Metrics and Logging system

[raduweiss]

• Merge logger code into firecracker (additional changes requests)
• Include timestamp in logging messages
• Add API support for attaching a log file (currently we only support it through vmm-no-api).
• Write the additional code that is necessary for collecting the various metrics
• Add API support for specifying a different file for the metrics

[dianpopa]

• Switch from syslog to some hostfile-backed mechanism #214
• Logging updates prior to supporting configuration through the API #246
• Specify log file through API #250
• Add support for specifying different files for flushing metrics #339.