Make higher precision v7 timestamps use 10 bits on platforms with microsecond precision #814
Description
See: #809 (comment)
In PostgreSQL, UUID V7 generation uses the rand_a field for an extra 12 bits of sub-millisecond timestamp precision. Some platforms don't generate timestamps with enough precision to fill that 12 bits though. In those cases, we can instead fill 10 bits, and leave another 2 free for random data.
This should be fairly straightforward to support in ContextV7, since additional precision is already pluggable and not fixed at 0 or 12 bits.