VER: Release 0.26.0

[threecgreen]

Enhancements

• Added v3 namespace in preparation for future DBN version 3 release. DBN version 2
  remains the current and default version
• Added v3::InstrumentDefMsg record with new fields to support normalizing multi-leg
  strategy definitions
  • Removal of statistics-schema related fields trading_reference_price,
    trading_reference_date, and settl_price_type
  • Removal of the status-schema related field md_security_trading_status
• Added from_instrument_def_v1_to_v3 and from_instrument_def_v2_to_v3 conversion
  functions to the C API
• Updated the value of the MAX_RECORD_LEN constant for the changes to
  InstrumentDefMsg in version 3
• Added initial support for merging DBN:
  • Decoding streams: MergeDecoder and MergeRecordDecoder structs
  • Metadata: MergeDecoder struct and Metadata::merge() method
  • In the CLI: specify more than one input file to initiate a merge
• Relaxed DecodeRecord trait constraint on StreamIterDecoder's inner decoder
• Added DbnMetadata implementation for StreamInnerDecoder if the inner decoder
  implements DbnMetadata
• Eliminate unsafe in From implementations for record structs from different versions

[nmacholl]

LGTM 🚀