Allow mod_dav_fs to tolerate race conditions in PROPFIND #389

Job	Run time
Empty APLOGNO() test	32s
Default	3m 54s
All-static modules	3m 58s
Prefork MPM, all-modules (except cgid)	4m 36s
Worker MPM, all-modules	5m 48s
Shared MPMs, all-modules	4m 39s
Event MPM, all-modules, mod_cgid only	5m 20s
Event MPM, all-modules, no CMSG_DATA	5m 26s
Default, all-modules + install	4m 51s
Default, all-modules, random test order	5m 56s
GCC 10 maintainer-mode w/-Werror, install + VPATH	4m 0s
All-modules, APR 1.7.4, APR-util 1.6.3	5m 38s
APR 1.8.x, APR-util 1.7.x	7m 37s
Pool-debug	8m 22s
Shared MPMs (event), pool-debug, SSL/TLS variants	11m 12s
Shared MPMs (worker), pool-debug, SSL/TLS variants	11m 17s
Shared MPMs (prefork), pool-debug, SSL/TLS variants	12m 17s
litmus WebDAV tests	3m 11s
Regenerate ap_expr	4m 23s
APR 1.7.4, APR-util 1.6.3, LDAP	4m 16s
APR trunk thread debugging	5m 8s
UBSan	5m 16s
ASan	8m 10s
ASan, pool-debug	10m 50s
HTTP/2 test suite	12m 36s
MOD_TLS test suite	5m 45s
	2h 44m 58s

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