CEP-15: Multi-Partition Transaction CQL Support (Alpha -> v1)

With CASSANDRA-17719 complete, basic/alpha CQL support for Accord is now available in the cep-15-accord branch. There are, however, a number of features and enhancements that we would still like to add before we make it available to users. Here is a summary of those items, broadly categorized:

Error Messages and UX

• Fix the element/field names in result metadata to match the user query
• Better error messages
  • Append/subtracting to/from frozen list
  • attempts to use reference in WHERE clause
  • LET using a reserved word

Domain Object Cleanup

• Given Selector is serializable, attempt to reuse it in TxnReference rather than hard coding data conversion cases
• Separate current current LET statement from SelectStatement, likely as a new LetStatement that names a SelectStatement
• Explore unifying Operation and ReferenceOperation
• Include non-Row items of FilteredPartition in TxnData#estimatedSizeOnHeap()
• Explore possibility of making Bound serializable and using it directly in TxnCondition
• Change internal representation of TxnDataName to avoid String -> ByteBuffer conversions
• Have TxnCondition/TxnUpdate support slice() to full take advantage of partial state replication
• Update serialization logic to reflect CASSANDRA-18099

Testing

• Figure out the original intent of placeholder tests in AccordIntegrationTest (ex. acceptInvalidationTest())
• Remove hack in AccordConfigurationService (when we have Transactional Metadata)
• Verify concretely that we disable Guardrails.
• Audit the way we propagate timestamps through execution.

Features

• Full JSON support
• Final decision of whether we should support returning the result of an IF/condition
• Constant terms on the RHS of LET
• Support for having a reference on both LHS and RHS of IF predicates
• Support for CONTAINS, CONTAINS KEY, and IN (important for CAS parity)
• Nested UDT support
• Mixed conditional and unconditional updates (should this be post-v1?)
• Arithmetic operations on references in INSERT/UPDATE (ex. INSERT INTO ks.tbl1 (k, c, v) VALUES (0, 0, a.v + 1)

Tooling

• Eliminate nodetool createepochunsafe once transactional metadata is available.

Documentation

• CQL language documentation and bump the CQL language specification version
• JavaDoc for all new statement classes
• Integrate demo.txt into broader documentation somewhere or remove

Questions

• How should txn statement parsing/preparation handle TTLs specified in updates?
• At what level do we want to integrate tracing?