* Allow global password cache to be turned off

The global password cache is undesirable for higher security applications as a user's cache entry would remain even if the calling package deleted the corresponding `UsmSecurityParameters`.

Disabling this mechanism comes with a significant performance hit, as seen by running the added benchmark tests with the build tag `gosnmp_nopwdcache`.

There is also a middle ground approach, where the calling package can reset the cache by turning it off then on again. This should clear any stale entries and still allow the cache to be leveraged.

Signed-off-by: Liam Kelly <liam@commdevices.com>