Improve ColumnWriteBuffer recycling

[AndreyNudko]

Return buffers to the pool after executing prepared INSERT statement, so they can be reused again even if statement is not closed

[AndreyNudko]

One [I think fairly common] pattern for uploading data to db is something akin to the following pseudo-code

while(isRunnning) {
  try (Connection con = openConnection();
       PreparedStatement stmt = prepareInsertStatement(con))
  {
    while(isRunning) {
      pollSomeQueueAndBatchNextMessage(stmt);
      if (timeToWriteToDb()) {
        stmt.executeUpdate();
      }
    }
  } catch(SQLException e) {
    handleError(e);
  }
}

Essentially you prepare statement once and keep using it until there's some connection error, in which case you throw current connection away and start again.
It turns out even after PR #382 this pattern still causes some memory churn.
What happens is that executeUpdate() sets blockInit=false, then re-initilizes existing block; this works ok - re-init returns buffer to the pool and immediately obtains it from pool again.
However next setObject() calls initBlockIfPossible(), which throws current block away w/o returning ColumnWriterBuffer to the pool
It then creates new block, but now pool is empty and new ColumnWriteBuffer-s have to be instantiated
This PR improves ColumnWriteBuffer recycling by returning them to the pool when marking block as uninitialized
This should improve situation for code like above, as well as for cases when JDBC connection pool implements statement caching.

[codecov]

Codecov Report

Merging #395 (bb66e4b) into master (830d40e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #395   +/-   ##
=========================================
  Coverage     62.43%   62.43%           
  Complexity     1250     1250           
=========================================
  Files           135      135           
  Lines          6640     6640           
  Branches        516      516           
=========================================
  Hits           4146     4146           
  Misses         2219     2219           
  Partials        275      275           

Impacted Files	Coverage Δ
...c/statement/ClickHousePreparedInsertStatement.java	53.21% <100.00%> (+0.30%)	⬆️
...housepower/jdbc/statement/ClickHouseStatement.java	54.65% <100.00%> (+0.53%)	⬆️
...rc/main/java/com/github/housepower/data/Block.java	89.70% <0.00%> (-1.48%)	⬇️
.../com/github/housepower/buffer/ByteArrayWriter.java	100.00% <0.00%> (ø)

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[sundy-li]

LGTM, thanks!