Improve MultiHttpClient connection concurrency and reuse

Use CURLMOPT_MAX_HOST_CONNECTIONS to enforce concurrent request limits.
This gives better concurrency than using naïve array_chunk() batches, which
were serialized and treated all URLs as pessimistically from the same host.

Allow connection reuse for multi-URL request batches. This avoids overhead
from reconnections and reduces the number of TIME_WAIT handles when many
batch operations happen in a short time frame. Previously, the use of the
CURLOPT_FORBID_REUSE flag meant that connections were cached but never
reused for multi-URL batches (only single-URL batches).

Connection limits can be verified by running large runMulti() batches
in an interactive shell and inspecting netstat for TCP connections.

Bug: T232128
Change-Id: I5c5f1eceb3fdb501a8f22f2b949756065f12379a