RFR: 8374808: Add an Instance in place of Date for KeyStore and KeyStoreSpi

[Mikhail Yankelevich]

Hi!

This is my proposal to transfer `KeyStore` and `KeyStoreSpi` with internal implementations to use `Instance`s instead of `Date`s. 
I would be very grateful for your comments and suggestions.

Thanks!

P.S. this is related to [JDK-8350953](https://bugs.openjdk.org/browse/JDK-8350953)

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Commit messages:
 - renamed timestamp to instant
 - renamed timestamp to instant
 - test
 - test
 - comments
 - Weijun's comments
 - JDK-8374808: Attempt to add an Instance in place of Date for KeyStore and KeyStoreSpi

Changes: https://git.openjdk.org/jdk/pull/29140/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29140&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374808
  Stats: 268 lines in 8 files changed: 197 ins; 4 del; 67 mod
  Patch: https://git.openjdk.org/jdk/pull/29140.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29140/head:pull/29140

PR: https://git.openjdk.org/jdk/pull/29140