This is a rollback of 0927cf739f
We cannot do that change until we either completes #9570 or possibly also use the new TryLock in GO 1.18.
Fixes#9588
Opens #4086
The change in lock logic for `partialCached` in 0927cf739f was naive as it didn't consider cached partials calling other cached partials.
This changeset may look on the large side for this particular issue, but it pulls in part of a working branch, introducing `context.Context` in the template execution.
Note that the context is only partially implemented in this PR, but the upcoming use cases will, as one example, include having access to the top "dot" (e.g. `Page`) all the way down into partials and shortcodes etc.
The earlier benchmarks rerun against master:
```bash
name old time/op new time/op delta
IncludeCached-10 13.6ms ± 2% 13.8ms ± 1% ~ (p=0.343 n=4+4)
name old alloc/op new alloc/op delta
IncludeCached-10 5.30MB ± 0% 5.35MB ± 0% +0.96% (p=0.029 n=4+4)
name old allocs/op new allocs/op delta
IncludeCached-10 74.7k ± 0% 75.3k ± 0% +0.77% (p=0.029 n=4+4)
```
Fixes#9519
This commit revises the locking strategy for `partialCached`. We have added a benchmark that may be a little artificial, but it should at least show that we're not losing any performance over this:
```bash
name old time/op new time/op delta
IncludeCached-10 12.2ms ± 2% 11.3ms ± 1% -7.36% (p=0.029 n=4+4)
name old alloc/op new alloc/op delta
IncludeCached-10 7.17MB ± 0% 5.09MB ± 0% -29.00% (p=0.029 n=4+4)
name old allocs/op new allocs/op delta
IncludeCached-10 128k ± 1% 70k ± 0% -45.42% (p=0.029 n=4+4)
```
This commit also revises the template metrics hints logic a little, and add a test for it, which output is currently this:
```bash
cumulative average maximum cache percent cached total
duration duration duration potential cached count count template
---------- -------- -------- --------- ------- ------ ----- --------
163.334µs 163.334µs 163.334µs 0 0 0 1 index.html
23.749µs 5.937µs 19.916µs 25 50 2 4 partials/dynamic1.html
9.625µs 4.812µs 6.75µs 100 50 1 2 partials/static1.html
7.625µs 7.625µs 7.625µs 100 0 0 1 partials/static2.html
```
Some notes:
* The duration now includes the cached invocations (which should be very short)
* A cached template gets executed once before it gets cached, so the "percent cached" will never be 100.
Fixes#4086Fixes#9506
