mysql: Batch select queries when required.

[lukebp]

Closes #1608

This commit updates the mysql package to batch SELECT queries if the
number of records being requested exceedes the MySQL limit for the
maximum number placeholders that can be included in a prepared
statement (65,535 placeholders).

As a side note, manually testing this commit required loading a tlog
tree with >65,535 leaves. This was surprisingly difficult to do and
highlights how we're using tlog in a way that it's not really built for.
We will need to write our own append-only log implementation at some
point in order to remove the performance bottlenecks that tlog has.

Log statements from the manual testing are shown below.

2021-12-20 09:04:55.964 [DBG] STOR: Get 83698 blobs using 2 prepared statements
2021-12-20 09:04:55.965 [DBG] STOR: Executing select statement 1/2
2021-12-20 09:04:56.925 [DBG] STOR: Executing select statement 2/2

[amass01]

tACK.