backupccl: add BackupMonitor to memory monitor file stitching

[adityamaru]

This change adds a BackupMonitor that hangs off the bulk memory
monitor. This monitor currently only guards the queue that we use
to buffer SSTs while stitching them together in the sstSink.

Informs: #73815

Release note: None

[cockroach-teamcity]

This change is 

[dt]

I don't immediately see why we need all the nil check no-ops on the accumulator but otherwise looks great

[adityamaru]

I do not understand the Bazel timeouts in testrace and stress:

[Run stress tests] _cgo_callers: relocation target x_cgo_callers not defined
[11:37:21][Run stress tests] _cgo_init: relocation target x_cgo_init not defined
[11:37:21][Run stress tests] _cgo_mmap: relocation target x_cgo_mmap not defined
[11:37:21][Run stress tests] _cgo_munmap: relocation target x_cgo_munmap not defined
[11:37:21][Run stress tests] _cgo_notify_runtime_init_done: relocation target x_cgo_notify_runtime_init_done not defined
[11:37:21][Run stress tests] _cgo_sigaction: relocation target x_cgo_sigaction not defined
[11:37:21][Run stress tests] _cgo_thread_start: relocation target x_cgo_thread_start not defined
[11:37:21][Run stress tests] external/go_sdk/pkg/tool/linux_amd64/link: too many errors
[11:37:21][Run stress tests] unexpected fault address 0x7f219d6c8000
[11:37:21][Run stress tests] fatal error: fault
[11:37:21][Run stress tests] unexpected fault address 0x7f219d61b000
[11:37:21][Run stress tests] fatal error: fault
[11:37:21][Run stress tests] link: error running subcommand external/go_sdk/pkg/tool/linux_amd64/link: exit status 2
[11:37:21][Run stress tests] Target //pkg/ccl/backupccl:backupccl_test failed to build
[11:37:21][Run stress tests] Use --verbose_failures to see the command lines of failed build steps.
[11:37:21][Run stress tests] INFO: Elapsed time: 75.045s, Critical Path: 46.72s
[11:37:21][Run stress tests] INFO: 795 processes: 317 internal, 478 processwrapper-sandbox.
[11:37:21][Run stress tests] FAILED: Build did NOT complete successfully
[11:37:21][Run stress tests] FAILED: Build did NOT complete successfully
[11:37:21][Run stress tests] ERROR: exit status 1

Rebased to try and kickoff another run to see if that fixes it.

[adityamaru]

Bazel team is aware of the linker error. Going to go ahead and bors this.

TFTR!
bors r=dt

[craig]

Build failed:

• GitHub CI (Cockroach)

[adityamaru]

TestBenchmarkExpectation/DropDatabase/drop_database_0_tables looks like a flake.

bors r=dt

[craig]

Build failed:

• GitHub CI (Cockroach)

[adityamaru]

Lets try this again, bazel failure is the known linker error.

bors r=dt

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 1ac3e93 to blathers/backport-release-21.2-73805: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2.x failed. See errors above.

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