Support multiple SD in RFC5424 (fix issue #61)

[vche]

*Issue #61 *

Description of changes:

• The log record structure has been updated to have structured data (SD) field as a vector instead of a single instance, in order to be able to hold multiple structured data
• Every decoder has been updated to store a vector with the processed SD (but only RFC5424 logs can have more than 1)
• RFC5424 decoder has been updated to parse every SD, in case multiple are specified, instead of only the first one
• Every encoder has beem updated to output every SD entry, whenever possible (capnp records can only hold 1, gelf and lstsv have limitation when the same key is specified in mutiple SD).
• Unit tests added for every encoder/decoder to validate the multiple SD handling
• Small typos and cosmetic changes
• Version update

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[kurojishi]

LGTM