Store `io_context` as shared pointer #1026

Job	Run time
Linux [UBSAN] [lmdb \| clang]	39m 29s
Linux [ASAN] [lmdb \| clang]	26m 35s
macOS [UBSAN] [lmdb \| clang]	34m 57s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	12m 23s
macOS [ASAN] [lmdb \| clang]	30m 50s
macOS [TSAN] [lmdb \| clang] (Errors Ignored)	31m 56s
Linux [TSAN] [lmdb \| clang] (Errors Ignored)	38m 7s
macOS [UBSAN] [rocksdb \| clang]	43m 52s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	27m 10s
macOS [ASAN] [rocksdb \| clang]	39m 44s
macOS [TSAN] [rocksdb \| clang] (Errors Ignored)	51m 9s
Linux [UBSAN] [rocksdb \| clang]	48m 46s
Linux [ASAN] [rocksdb \| clang]	27m 29s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	32m 26s
Linux [TSAN] [rocksdb \| clang] (Errors Ignored)	18m 42s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	52m 26s
	9h 16m 1s

Provide feedback