Fixed #23157 -- Improved file upload performance when many duplicate names exist

Storage.get_available_name() previously handled naming conflicts by
exhaustively testing each possible alternative, starting at one until a
free name was found.

In some scenarios this would lead to pathological performance due to
stat()ing a huge number of existent files, potentially causing I/O and
reducing cache effectiveness by warming the metadata of previously cold
files.

This patch preserves the existing naming scheme (incrementing integers),
but introduces a binary search to find the first free name. Given a
filename that is duplicated 10,000 times, the number of stat() calls
required to find a free name is reduced from 10,000 to 24.

    ['highest', None, 'got', 0, 'num stats', 1]
    ['highest', 0, 'got', 1, 'num stats', 6]
    ['highest', 1, 'got', 2, 'num stats', 6]
    ['highest', 2, 'got', 3, 'num stats', 6]
    ['highest', 3, 'got', 4, 'num stats', 6]
    ['highest', 4, 'got', 5, 'num stats', 6]
    ['highest', 5, 'got', 6, 'num stats', 6]
    ['highest', 6, 'got', 7, 'num stats', 6]
    ['highest', 7, 'got', 8, 'num stats', 6]
    ['highest', 8, 'got', 9, 'num stats', 6]
    ['highest', 9, 'got', 10, 'num stats', 6]
    ['highest', 10, 'got', 11, 'num stats', 7]
    ['highest', 100, 'got', 101, 'num stats', 12]
    ['highest', 1000, 'got', 1001, 'num stats', 18]
    ['highest', 10000, 'got', 10001, 'num stats', 24]
    ['highest', 100000, 'got', 100001, 'num stats', 32]
    ['highest', 1000000, 'got', 1000001, 'num stats', 38]
    ['highest', 10000000, 'got', 10000001, 'num stats', 44]

This should be compatible with the old implementation, except that if
random parts of the conflicting range become free again due to deletion,
the original implementation would always pick them first. This version
cannot guarantee the free parts are noticed, and it doesn't appear
possible without reintroducing pathological behavior.

Ideally by default Django would use some randomized naming scheme,
although this might be a surprising change for existing installations
(especially since e.g. intuitive directory sorting is broken).

Time-based names are another option, but are likely to break for high
traffic / multi-server installations.