RFR: 8271525: ResourceHashtableBase::iterate() should not declared as const

[Ioi Lam]

`ResourceHashtableBase::iterate()` is declared `const`, but it can actually change the contents of the table. The same is true for `ResourceHashtableBase::get()`, which returns a non-`const` pointer to the value, allowing the caller to modify it.

We should declare these two functions as non-`const`. This will also remove a lot of ugly `const_cast<>` code.

The `iterate()` API is tightened such that the `do_entry()` function can modify the `value` but not the `key`.

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Commit messages:
 - added const_iterate()
 - added const version of get()
 - fixed spacing
 - fixed typos
 - also make the get() function non-const, since it returns a pointer to the value, which the caller can modify
 - step2 -- make iterate() a non-const function
 - step1 -- keep iterate() as a const function, but clean up the rest of the code

Changes: https://git.openjdk.java.net/jdk/pull/4942/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4942&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271525
  Stats: 60 lines in 6 files changed: 29 ins; 4 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4942.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4942/head:pull/4942

PR: https://git.openjdk.java.net/jdk/pull/4942