The `_CONSOLE_API_MSG` buffer is resized to cover an entire message.
Later on any UTF-8 data is cached in a separate temporary
buffer inside `til::u8state` to prevent lone surrogate pairs.
Both cases are problematic as neither buffer is freed after the read
has finished. Passing a 100MB buffer to conhost once will thus cause it
to continue using ~220MB of physical memory until the conhost process exits.
This change releases unneeded memory as soon as the requested buffer
size has halved. In practice this means that once a command has returned
all buffers will shrink, as the shell commonly sends very small messages.
## PR Checklist
* [x] Closes#10731
* [x] I work here
* [x] Tests added/passed
## Validation Steps Performed
* Buffers aren't reallocated during printing ✔️
* Buffers shrink after printing finished ✔️
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