Make parameters to SensorDataStore functions const where possible. Requires recompiling of binaries that depend on libdcdb!

- Allows to set a variety of conditions on sensor data and raise
alarms when they are triggered
- Arbitrary scripts can be executed (e.g., to send emails) when
alarms happen

- Also added support for multiple accept sockets when a single hostname
maps to multiple IPs

- Now using the global thread pool in dcdbpusher, collectagent and
grafanaserver to boost performance

- Fixing warnings here and there

- Plus other logging consistency changes

- SensorMetadata objects do not rely on shared pointers anymore, results
in a 50% (or more) reduced memory usage
- Sensor cache files are written as CSV files, bypassing BOOST's PTREE
parsing
- With a local DB instance, retrieving 100k sensors from the cache is
now between 2X and 3X faster than querying Cassandra

- Keeps track of sensor metadata in a JSON file within .cache
- Implemented at the libdcdb level
- Makes use of a lock file to prevent concurrency issues
- Can be enabled via the USE_SENSOR_CACHE compile flag

- Uses a simple string key->value schema
- Stores the timestamp of the latest edit to the publishedsensors table
- Implemented logic to update this timestamp accordingly