Request to add a custom comparators

[fmvin]

There are tasks to read timeseries data from primary rocksdb stored in special column family.
Would be nece to have such extra comparator. For example, to create a predefined comparator to work with timestamp keys.

[Congyuwang]

How is your timestamp structured, and what specific comparator logic do you need (bitwise)? And also, if this is to be implemented, what are some of the commonly used comparators would you suggest to be added? Thanks.

[fmvin]

My timestamp is an int64 value representing a Unix timestamp in milliseconds. The user key consists of a string with an 8-byte timestamp appended as a suffix.

The suggestion is to implement the following comparators, as described in comparator.h:

• BytewiseComparatorWithU64Ts
• ReverseBytewiseComparatorWithU64Ts