Added an ImapToken cache to try and reduce ImapToken allocations

[jstedfast]

Theoretically, this could drastically reduce ImapToken allocations which might help with issue #1335.

Of course, this also adds more overhead in other aspects.

TODO:

• Write unit tests for ImapTokenCache
• Play with different cache limits to see if there is an optimal limit
• Move the cache to be ImapEngine-specific? Might be better than a global ImapToken cache like it is now. Another advantage of that would be that the mutex locking wouldn't be necessary anymore. Not sure how expensive locking is...
• Benchmark?

Alternative Idea:

Instead of (or in combination with?) the ImapTokenCache, if we used ReadOnlyMemory<byte>, we wouldn't need:

1. to allocate a token buffer
2. to copy the token into the token buffer

This would reduce memory allocations, memory fragmentation, and improve performance because there'd be no (or less) need to copy data around.

That said, there are a few things to consider:

1. QString tokens would still need to be copied (because we unquote them as we copy them) or we would need to change the code that handles qstring tokens to unquote as they are consumed.
2. long tokens that require re-filling the buffer would add complexity and, in worst-case scenarios, might end up with tokens longer than the I/O input buffer, requiring us to allocate a "token buffer" anyway. It should be exceptionally rare since the I/O buffer is 4K, but we still need to handle this scenario somehow - even if it is to just throw an exception (which may be a perfectly legitimate thing to do).
3. The current IMAP implementation has situations where we can unget a series of tokens (I think up to 3?) which would be risky since it's possible a second or third token would require refilling the buffer which would "corrupt" the older token(s). We need to handle this somehow.

[coveralls]

coverage: 92.78% (-0.09%) from 92.869%
when pulling 90db485 on imap-token-cache
into 8479581 on master.

[jstedfast]

How much, in terms of allocations, does the ImapTokenCache save us?

No cache (for comparison):

With this PR (cache capacity = 128):

capacity = 256: