[analyzer] Cache deserialized unlinked units, don't deserialize the same units over and over again

TL;DR: In the scenario outlined below this CL makes the analyzer use
~50MB less ram after initial analysis, answer the first implements query
~1.2 seconds (~25%) faster and use ~300MB less ram after the first
implements query.

Details:

The below measurements are taken with 8 folders from the sdk open that
overlaps: `pkg/analysis_server`, `pkg/analysis_server_client`,
`pkg/analyzer`, `pkg/analyzer_cli`, `pkg/analyzer_plugin`,
`pkg/analyzer_utilities`, `pkg/_fe_analyzer_shared`, `pkg/front_end`).

The query run is a find all implementations query for the class
`ToJsonable` in
`pkg/analysis_server/lib/lsp_protocol/protocol_special.dart`.

An initial run not shown here was made to let the analyzer cache results as needed.

BEFORE:

isAnalyzing is now done after 0:00:06.661205
heap 524.6MB of 546.8MB
Got answer to query in 0:00:05.285724
heap 943.8MB of 990.1MB

isAnalyzing is now done after 0:00:06.766694
heap 525.0MB of 547.0MB
Got answer to query in 0:00:04.932227
heap 941.9MB of 990.5MB

isAnalyzing is now done after 0:00:06.806875
heap 522.4MB of 547.6MB
Got answer to query in 0:00:05.227859
heap 941.9MB of 990.5MB

isAnalyzing is now done after 0:00:06.663847
heap 507.4MB of 550.0MB
Got answer to query in 0:00:05.107593
heap 941.9MB of 991.0MB

isAnalyzing is now done after 0:00:06.649731
heap 507.4MB of 549.8MB
Got answer to query in 0:00:05.055295
heap 941.9MB of 990.5MB


WITH CL:

isAnalyzing is now done after 0:00:06.575444
heap 461.7MB of 489.2MB
Got answer to query in 0:00:03.758280
heap 628.0MB of 671.6MB

isAnalyzing is now done after 0:00:06.630200
heap 471.2MB of 488.9MB
Got answer to query in 0:00:03.786806
heap 643.3MB of 671.6MB

isAnalyzing is now done after 0:00:06.437794
heap 465.0MB of 485.6MB
Got answer to query in 0:00:03.995996
heap 628.0MB of 671.3MB

isAnalyzing is now done after 0:00:06.706091
heap 457.4MB of 487.2MB
Got answer to query in 0:00:03.748728
heap 640.8MB of 671.1MB

isAnalyzing is now done after 0:00:06.610832
heap 467.7MB of 486.2MB
Got answer to query in 0:00:03.877194
heap 643.9MB of 671.1MB


Initial analysis:
No difference proven at 95.0% confidence

After initial analysis, heap, utilized:
Difference at 95.0% confidence
        -52.76 +/- 10.9161
        -10.1979% +/- 2.10996%
        (Student's t, pooled s = 7.48475)

After initial analysis, heap, capacity:
Difference at 95.0% confidence
        -60.82 +/- 2.29212
        -11.0937% +/- 0.418087%
        (Student's t, pooled s = 1.57162)

First implements query:
Difference at 95.0% confidence
        -1.28834 +/- 0.18012
        -25.1543% +/- 3.51677%
        (Student's t, pooled s = 0.123502)

After first implements query, heap, utilized:
Difference at 95.0% confidence
        -305.48 +/- 8.41655
        -32.4192% +/- 0.893211%
        (Student's t, pooled s = 5.77092)

After first implements query, heap, capacity:
Difference at 95.0% confidence
        -319.18 +/- 0.418906
        -32.2235% +/- 0.0422915%
        (Student's t, pooled s = 0.287228)

Change-Id: Ice2bd0e8c90285c2097afe2589d19f79762d3837
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/275201
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>