Don't serialize returning connections to pool in background

[bgrainger]

Greatly improves latency for #979.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

0.0% Coverage
0.0% Duplication

[bgrainger]

A test was run with wrk -t4 -c100 -d60s -R2000 --latency 'http://localhost:5000/...'

Legend: CR = ConnectionReset; DCR = DeferConnectionReset. Columns (left to right):

• ConnectionReset=false should be the fastest, as it eliminates the reset work entirely
• DeferConnectionReset=true is the behaviour of 1.2.x; should be slower due to extra work (resetting) when opening a connection
• DeferConnectionReset=false is default behaviour in 1.3.x; lowers initial latency but can get very bad
• This PR is the same settings as column 3, but with code to improve latency; it's extremely competitive with column 1 at the cost of increased concurrent connection usage to reset the connections in the background

	CR = false	CR=true;DCR=true	CR=true;DCR=false	This PR
Max Connections	100	100	200	182
P50	19.07	27.60	18.82	18.77
P75	19.63	28.06	19.25	19.17
P90	20.43	28.59	19.74	19.58
P99	23.57	31.63	39.26	23.92
P99.9	35.04	38.14	164.99	36.64
P99.99	166.14	220.16	865.28	76.22
P99.999	230.01	247.55	1,150.00	235.65
P100	230.14	256.51	1,180.00	235.65

[bgrainger]

Fixed in 1.3.8.