fix(in-memory): resolve orphaned skiplist arena leak on Close

[parthbhanti22]

Description

Fixes #2205.

Root Cause:
When Badger is configured with opt.InMemory = true, the db.close() function triggers an early return. This prevents the cleanup logic from iterating over the immutable memtables slice (db.imm). Because mmap allocations bypass Go's GC, failing to explicitly call DecrRef() on these memtables leaves the skl.Arena allocations orphaned in memory indefinitely.

The Fix:
Added an explicit iteration over db.imm to decrement reference counts on all immutable memtables prior to the early return in the in-memory shutdown sequence.

Verification:
Included issue_2205_test.go to aggressively cycle in-memory database instantiation.

• Before: pprof inuse_space showed ~83GB of retained skl.newArena allocations across 1000 iterations.
• After: Retained arenas dropped to 0 bytes.

All tests in ./test.sh pass locally.

Checklist

• Code compiles correctly and linting passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable

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