RFR: Update nmethod caching and clean up unused/duplicate methods [v8]

[Ashutosh Mehra]

> This PR updates nmethod caching to be based on caching of CodeBlob. It also cleans up some dead code and duplicate methods.

Ashutosh Mehra has updated the pull request incrementally with one additional commit since the last revision:

  Add back CodeBuffer::clear_strings()
  
  Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

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Changes:
  - all: https://git.openjdk.org/leyden/pull/71/files
  - new: https://git.openjdk.org/leyden/pull/71/files/446ac35f..31e581d4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=leyden&pr=71&range=07
 - incr: https://webrevs.openjdk.org/?repo=leyden&pr=71&range=06-07

  Stats: 7 lines in 2 files changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/leyden/pull/71.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/71/head:pull/71

PR: https://git.openjdk.org/leyden/pull/71