T84900: Use a JS Object for storing conversion functions

Until now, conversion functions were stored in an array of JS Objects,
which is sub-optimal since in 99% of the cases where a conversion is
needed, the attribute's name is known in advance. The new outline allows
one to invoke the conversion function directly by using
schema.conversions.<attName>.(write|read)().
T84900: Remove all partial conversions from the code

There were some places in the code where some partial solutions to
conversion were present. This commit removes these portions, as they do
not represent a systematic approach to value conversion.
T84900: Apply conversions systematically to build(Get|Put)Query()

This commit presents a systematic way of converting values from JS
Objects into Cassandra values. The approach is rather simple: every time
there is a query construction under way, remember the parameters'
attribute names as well (in order they are appended). Then, before
returning the query's CQL and parameters, convert them using the
schema.conversions JS Object containing the conversion functions.
T84900: Convert values when deleting rows as well
Minor: add 'neq' as a synonym for 'ne'
T84900: Ensure the proper conversion in secondaryIndexes.js

Whenever a row is updated, the indices get rebuilt by comparing the new
version with the old one. As this involves readings from Cassandra, the
values provided need to be properly converted.