Reduce memory hoarding in the unit test

[bbockelm]

This PR reduces memory hoarding and memory churn by:

1. Hoisting a regexp compilation from a local (called from the HTTP request handler) to a global.
2. Deleting an entry out of a cache once we will no longer use it.
3. Providing the unit tests with a configuration knob to completely disable local xrootd monitoring, making their memory usage more predictable.

[brianaydemir]

I modified the test to run for 10,000 iterations, and collected one profile without this PR's changes (mem.before), and one profile with them (mem.after).

The reduction in allocations:

# go tool pprof -nodefraction 0 -alloc_space -unit megabyte -diff_base mem.before mem.after
File: director.test
Build ID: 8c833cb3bfda65d2a42fcab1e017b1fce61c1442
Type: alloc_space
Time: Feb 19, 2025 at 4:34pm (UTC)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for -330.97MB, 11.25% of 2940.82MB total
Showing top 10 nodes out of 994
      flat  flat%   sum%        cum   cum%
 -103.05MB  3.50%  3.50%  -103.05MB  3.50%  regexp/syntax.(*compiler).inst
  -60.01MB  2.04%  5.54%   -60.01MB  2.04%  regexp/syntax.(*parser).newRegexp
     -46MB  1.56%  7.11%  -185.52MB  6.31%  github.com/sirupsen/logrus.(*TextFormatter).Format
  -43.01MB  1.46%  8.57%   -43.01MB  1.46%  strconv.appendQuotedWith
  -28.50MB  0.97%  9.54%   -71.51MB  2.43%  fmt.Sprintf
  -18.49MB  0.63% 10.17%   -18.49MB  0.63%  bytes.growSlice
  -16.50MB  0.56% 10.73%  -153.52MB  5.22%  github.com/pelicanplatform/pelican/metrics.GetSIDRest
  -14.91MB  0.51% 11.24%   -14.91MB  0.51%  strings.genSplit
  -14.50MB  0.49% 11.73%   -16.99MB  0.58%  regexp.(*Regexp).ReplaceAllString
      14MB  0.48% 11.25%       30MB  1.02%  vendor/golang.org/x/crypto/hkdf.Expand

The reduction in live heap:

# go tool pprof -nodefraction 0 -inuse_space -unit megabyte -diff_base mem.before mem.after
File: director.test
Build ID: 8c833cb3bfda65d2a42fcab1e017b1fce61c1442
Type: inuse_space
Time: Feb 19, 2025 at 4:34pm (UTC)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for -18.55MB, 69.91% of 26.53MB total
Showing top 10 nodes out of 111
      flat  flat%   sum%        cum   cum%
   -7.50MB 28.27% 28.27%    -7.50MB 28.27%  github.com/pelicanplatform/pelican/metrics.GetSIDRest
   -4.91MB 18.53% 46.80%    -9.06MB 34.15%  github.com/jellydator/ttlcache/v3.(*Cache[go.shape.struct { Prot string; User string; Pid int; Sid int; Host string },go.shape.struct { Id uint32 }]).set
   -2.50MB  9.43% 56.23%    -2.50MB  9.43%  github.com/jellydator/ttlcache/v3.newItem[go.shape.struct { Id uint32 },go.shape.struct { AuthenticationProtocol string; User string; DN string; Role string; Org string; Groups []string; Project string; Host string }]
   -2.50MB  9.43% 65.65%    -2.50MB  9.43%  github.com/jellydator/ttlcache/v3.newItem[go.shape.struct { Prot string; User string; Pid int; Sid int; Host string },go.shape.struct { Id uint32 }]
      -2MB  7.54% 73.19%       -2MB  7.54%  container/list.(*List).insertValue (inline)
    1.50MB  5.67% 67.52%     1.50MB  5.67%  github.com/aws/aws-sdk-go/aws/endpoints.init
      -1MB  3.77% 71.29%       -1MB  3.77%  regexp.onePassCopy
   -0.64MB  2.43% 73.72%    -0.64MB  2.43%  github.com/jellydator/ttlcache/v3.(*expirationQueue[go.shape.struct { Prot string; User string; Pid int; Sid int; Host string },go.shape.struct { Id uint32 }]).Push
    0.51MB  1.92% 71.81%     0.51MB  1.92%  encoding/xml.map.init.0
    0.50MB  1.89% 69.91%     0.50MB  1.89%  sync.(*Map).LoadOrStore