raw_timestamp_to_cache_version

Converts a raw database string to `:usec` format.

Example:

timestamp = "2018-10-15 20:02:15.266505"
raw_timestamp_to_cache_version(timestamp)
# => "20181015200215266505"

PostgreSQL truncates trailing zeros, github.com/postgres/postgres/commit/3e1beda2cde3495f41290e1ece5d544525810214 to account for this we pad the output with zeros