Discard mode

[umputun]

Add the Discard option to both.SizedGroup and ErrSizedGroup. With this option, when the group is "locked" (i.e., over-capacity), any invocation of Go will be silently discarded until the group is unlocked (i.e., under-capacity). This allows for more efficient concurrency handling in scenarios where it's acceptable to skip the execution of additional goroutines when the group has reached its maximum capacity.

[coveralls]

Pull Request Test Coverage Report for Build 4648404867

• 29 of 29 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 94.805%

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Totals
Change from base Build 4646752419:	0.7%
Covered Lines:	146
Relevant Lines:	154

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💛 - Coveralls